Fix pool not working bugs and autocommit

The original implementation do not get new connection from the pool so it is actually not working.
and when creating test db, there's always a 'CREATE DATABASE' in a transaction block error due to
the lack of reimplementation of `_set_autocommit`method for the proxied connection.

I tried to fix these two bugs and it is now working in Django 1.7, I think it should also work for older
version.