Use multi-row inserts for massive speedups on to_sql over high latency connections
I have been trying to insert ~30k rows into a mysql database using pandas-0.15.1, oursql-0.9.3.1 and sqlalchemy-0.9.4. Because the machine is as across the atlantic from me, calling `data.to_sql` was taking >1 hr to insert the data. On inspecting with wireshark, the issue is that it is sending an insert for every row, then waiting for the ACK before sending the next, and, long story short, the ping times are killing me.

However, following the instructions from [SQLAlchemy](http://docs.sqlalchemy.org/en/latest/core/dml.html#sqlalchemy.sql.expression.Insert.values), I changed 

``` python
def _execute_insert(self, conn, keys, data_iter):
    data = [dict((k, v) for k, v in zip(keys, row)) for row in data_iter]
    conn.execute(self.insert_statement(), data)
```

to 

``` python
def _execute_insert(self, conn, keys, data_iter):
    data = [dict((k, v) for k, v in zip(keys, row)) for row in data_iter]
    conn.execute(self.insert_statement().values(data))
```

and the entire operation completes in less than a minute. (To save you a click, the difference is between multiple calls to `insert into foo (columns) values (rowX)` and one massive `insert into foo (columns) VALUES (row1), (row2), row3)`). Given how often people are likely to use pandas to insert large volumes of data, this feels like a huge win that would be great to be included more widely.

Some challenges:
- Not every database supports multirow inserts (SQLite and SQLServer didn't in the past, though they do now). I don't know how to check for this via SQLAlchemy
- The MySQL server I was using didn't allow me to insert the data all in one go, I had to set the chunksize (5k worked fine, but I guess the full 30k was too much). If we made this the default insert, most people would have to add a chunk size (which might be hard to calculate, as it might be determined by the maximum packet size of the server).

The easiest way to do this, would be to add a `multirow=` boolean parameter (default `False`) to the `to_sql` function, and then leave the user responsible for setting the chunksize, but perhaps there's a better way?

Thoughts?
