read_sql chokes on mysql when using labels with queries due to unnecessary quoting
Not sure if this is a pandas bug or an upstream one, but here's an example of the bug (pandas-0.14.1, mariadb 10, sqlalchemy-0.9.4)

``` python
engine=create_engine('mysql://{username}:{password}@{host}/{database}?charset=utf8'.format(**db))
pandas.io.sql.read_sql('SELECT onlinetransactions.id FROM onlinetransactions LIMIT 1', engine)   #Does what you'd expect
pandas.io.sql.read_sql('SELECT onlinetransactions.id as firstid FROM onlinetransactions LIMIT 1', engine)  #Fails
```

The error you get back is:

``` python
C:\Anaconda\lib\site-packages\pandas\io\sql.pyc in read_sql(sql, con, index_col, coerce_float, params, parse_dates, columns)
    421             coerce_float=coerce_float, parse_dates=parse_dates)
    422 
--> 423     if pandas_sql.has_table(sql):
    424         pandas_sql.meta.reflect(only=[sql])
    425         return pandas_sql.read_table(

C:\Anaconda\lib\site-packages\pandas\io\sql.pyc in has_table(self, name)
    847 
    848     def has_table(self, name):
--> 849         return self.engine.has_table(name)
    850 
    851     def get_table(self, table_name):

C:\Anaconda\lib\site-packages\sqlalchemy\engine\base.pyc in has_table(self, table_name, schema)
   1757 
   1758         """
-> 1759         return self.run_callable(self.dialect.has_table, table_name, schema)
   1760 
   1761     def raw_connection(self):

C:\Anaconda\lib\site-packages\sqlalchemy\engine\base.pyc in run_callable(self, callable_, *args, **kwargs)
   1661         """
   1662         with self.contextual_connect() as conn:
-> 1663             return conn.run_callable(callable_, *args, **kwargs)
   1664 
   1665     def execute(self, statement, *multiparams, **params):

C:\Anaconda\lib\site-packages\sqlalchemy\engine\base.pyc in run_callable(self, callable_, *args, **kwargs)
   1188 
   1189         """
-> 1190         return callable_(self, *args, **kwargs)
   1191 
   1192     def _run_visitor(self, visitorcallable, element, **kwargs):

C:\Anaconda\lib\site-packages\sqlalchemy\dialects\mysql\base.pyc in has_table(self, connection, table_name, schema)
   2274         try:
   2275             try:
-> 2276                 rs = connection.execute(st)
   2277                 have = rs.fetchone() is not None
   2278                 rs.close()

C:\Anaconda\lib\site-packages\sqlalchemy\engine\base.pyc in execute(self, object, *multiparams, **params)
    710         """
    711         if isinstance(object, util.string_types[0]):
--> 712             return self._execute_text(object, multiparams, params)
    713         try:
    714             meth = object._execute_on_connection

C:\Anaconda\lib\site-packages\sqlalchemy\engine\base.pyc in _execute_text(self, statement, multiparams, params)
    859             statement,
    860             parameters,
--> 861             statement, parameters
    862         )
    863         if self._has_events or self.engine._has_events:

C:\Anaconda\lib\site-packages\sqlalchemy\engine\base.pyc in _execute_context(self, dialect, constructor, statement, parameters, *args)
    945                                 parameters,
    946                                 cursor,
--> 947                                 context)
    948 
    949         if self._has_events or self.engine._has_events:

C:\Anaconda\lib\site-packages\sqlalchemy\engine\base.pyc in _handle_dbapi_exception(self, e, statement, parameters, cursor, context)
   1106                                         self.dialect.dbapi.Error,
   1107                                         connection_invalidated=self._is_disconnect),
-> 1108                                     exc_info
   1109                                 )
   1110 

C:\Anaconda\lib\site-packages\sqlalchemy\util\compat.pyc in raise_from_cause(exception, exc_info)
    183             exc_info = sys.exc_info()
    184         exc_type, exc_value, exc_tb = exc_info
--> 185         reraise(type(exception), exception, tb=exc_tb)
    186 
    187 if py3k:

C:\Anaconda\lib\site-packages\sqlalchemy\engine\base.pyc in _execute_context(self, dialect, constructor, statement, parameters, *args)
    938                                      statement,
    939                                      parameters,
--> 940                                      context)
    941         except Exception as e:
    942             self._handle_dbapi_exception(

C:\Anaconda\lib\site-packages\sqlalchemy\engine\default.pyc in do_execute(self, cursor, statement, parameters, context)
    433 
    434     def do_execute(self, cursor, statement, parameters, context=None):
--> 435         cursor.execute(statement, parameters)
    436 
    437     def do_execute_no_params(self, cursor, statement, context=None):

C:\Anaconda\lib\site-packages\MySQLdb\cursors.pyc in execute(self, query, args)
    203             del tb
    204             self.messages.append((exc, value))
--> 205             self.errorhandler(self, exc, value)
    206         self._executed = query
    207         if not self._defer_warnings: self._warning_check()

C:\Anaconda\lib\site-packages\MySQLdb\connections.pyc in defaulterrorhandler(***failed resolving arguments***)
     34     del cursor
     35     del connection
---> 36     raise errorclass, errorvalue
     37 
     38 re_numeric_part = re.compile(r"^(\d+)")

ProgrammingError: (ProgrammingError) (1103, "Incorrect table name 'SELECT onlinetransactions.id as firstid FROM onlinetransactions LIMIT 1'") 'DESCRIBE `SELECT onlinetransactions.id as firstid FROM onlinetransactions LIMIT 1`' ()
```

So it never gets as far as running the actual query, because it's tried to run a `DESCRIBE` query with ``` quotes which fails. i.e.

```
MariaDB [transactions]> DESCRIBE `SELECT onlinetransactions.id as firstid FROM onlinetransactions LIMIT 1`;
ERROR 1103 (42000): Incorrect table name 'SELECT onlinetransactions.id as firstid FROM onlinetransactions LIMIT 1'
```

But

```
MariaDB [transactions]> DESCRIBE SELECT onlinetransactions.id as firstid FROM onlinetransactions LIMIT 1;
+------+-------------+--------------------+-------+---------------+--------------------------------+---------+------+----------+-------------+
| id   | select_type | table              | type  | possible_keys | key                            | key_len | ref  | rows     | Extra       |
+------+-------------+--------------------+-------+---------------+--------------------------------+---------+------+----------+-------------+
|    1 | SIMPLE      | onlinetransactions | index | NULL          | ix_OnlineTransactions_Rpt_Year | 5       | NULL | 11485535 | Using index |
+------+-------------+--------------------+-------+---------------+--------------------------------+---------+------+----------+-------------+
1 row in set (0.00 sec)
```

Ok. So looking at the stacktrace, I reckon this _is_ a pandas bug as it seems to be calling `has_table` on my sql query, which doesn't seem to make any sense?
