Params not properly supported for pymssql
Pandas read SQL functions internally convert params to a list in pandas.io.sql._convert_params().

However on pymssql, params must be a tuple or dictionary. Stacktrace below. 

```
Traceback (most recent call last):
  File "/Users/simon/.virtualenvs/rfcapital/lib/python3.4/site-packages/pandas/io/sql.py", line 1539, in execute
    cur.execute(*args)
  File "pymssql.pyx", line 447, in pymssql.Cursor.execute (pymssql.c:6274)
  File "_mssql.pyx", line 998, in _mssql.MSSQLConnection.execute_query (_mssql.c:10085)
  File "_mssql.pyx", line 1029, in _mssql.MSSQLConnection.execute_query (_mssql.c:9964)
  File "_mssql.pyx", line 1146, in _mssql.MSSQLConnection.format_and_run_query (_mssql.c:11010)
  File "_mssql.pyx", line 1168, in _mssql.MSSQLConnection.format_sql_command (_mssql.c:11219)
  File "_mssql.pyx", line 1799, in _mssql._substitute_params (_mssql.c:18045)
ValueError: 'params' arg (<class 'list'>) can be only a tuple or a dictionary.
```
