Adds auto-imports to shell_plus.

This uses django-extensions's SHELL_PLUS_POST_IMPORTS to auto-import some useful functions like Q, nest_Q, and so on.  It also bumps the django-extensions version to 1.2.5, since SHELL_PLUS_POST_IMPORTS only does anything in >=1.2.3.  The only known backwards-incompatibility in 1.2.* is to UUIDField, which we do not use, so this should be safe.

I added some things that I thought would be useful, but we can add more if people want more.