Django17 prep

Things we need for Django 1.7, which isn't much happily. I'll wait until Django 1.7 is official before merging and it will likely be the last step before releasing the new version of ArmSections.

**Test loading** is different due to changes in App registration. This delays the import of the support models so Django can traverse the module first and register them. (The warning is listed in the commit message.) We'll need to do this for other components that use test support models. (Note: `load_tests` is not available in Python 2.6 but we are already using a backport of `unittest2` via the Django test runner. 1.7 deprecates the backport, but doesn't support Python 2.6, so no worries.)