Improved support for custom user models

Support for custom user models with a non-standard username field
Fix user changelist for custom user models

Fixed the user_admin_changelist url for custom user models.
Typo in previous commit
Fix for no model_name in django < 1.6
Make sure custom user logic works with Django 1.4

Monkey Patch for Django 1.4 to allow new custom user model logic.
Revert "Make sure custom user logic works with Django 1.4"

This reverts commit 1aed6c9a967a91d724b3efdfdcd4e84b3070fc62.
Merge pull request #272 from Tyris/master

Improved custom user model support