Partial fix for mixed-case username bug [GH-564].

The "password-case-warning" branch is misnamed, and actually addresses a bug with mixed-case usernames [GH-564].

This does not solve the issue for the creation of single users; it just adds a warning to the page with the list of users. It solves the issue for bulk uploads by converting each username to lowercase after it is read in from the CSV.

* makahiki/apps/managers/player_mgr/player_mgr.py: Added code to convert usernames to lowercase for bulk uploads
* makahiki/templates/admin/auth/user/bulk_upload_form.html: Added warning about uppercase to lowercase conversion
* makahiki/apps/managers/player_mgr/admin.py: Added bolded warning text that appears at the top of the page that lists users