Add support for django 1.4+. Drop support for 1.3

This would need a carefull look by someone more experienced with connection-pooling software than myself. I did not review the changes (if any?) to django from 1.4 to 1.5  that may affect this package. I'm using it on production (about 500 calls per minute to the db) on django trunk and have not experienced any problems.

I also made a design decision to drop 1.3 support to keep the code size as small as possible. If you still want to support 1.3 I would be happy to make a new pull-request.