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].

It adds a warning to the page with the list of users, but does not solve the issue for the creation of individual 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