Fixed Django 1.5 custom User model support to prevent circular imports.

All tests pass, and if the tests are run within a project with a custom
User model, they will also pass after the permission names in the tests
are modified to use the custom model, and the fixtures are similarly
adjusted.
Fix test failures whne ran in projects that register the ContentType admin.
Updated AUTHORS to add myself, as requested.
Merge pull request #114 from ixc/master

Fixed custom User compatibility in Django 1.5, preventing circular imports.