In PostgresqlExtDatabase, respect the 'autorollback' option
Thanks for this!

First of all, thanks so much for Peewee! I ran across the problem described in #240, which was fixed by the addition of the `autorollback` database option.

I was setting `autorollback=True`, but still hitting the problem, and realized that the implementation of `execute_sql` in `PostgresqlExtDatabase` (which I'm using) doesn't include the check to see if the transaction should be automatically rolled back if the query throws an exception. I just copied in the two lines from the parent `Database` class that do this.
