Register the custom User model by extending UserAdmin

[aktech]

Ref: https://docs.djangoproject.com/en/1.8/topics/auth/customizing/#custom-users-and-django-contrib-admin

Issues Fixed:

• [x] Password not hashing on creating new user on admin site
• [x] pep8 (Leave two lines on class definition).

[aktech]

Also, we need to write test for all the apps we're building.

Yeah, Perhaps, I was thinking about the same. ;)