User model substitued

Extending the user model was not enough.
Extended the AbstractUser to Employee for later extension.
In addition, added views for UserList & UserDetail with serializers to
get the users' information.
Auth tests

auth\tests.py:
- AccountTests:
- test_create_account - a test to ensure we can create account with the
webAPI
- test_delete_account - a test to ensure we can delete an account with
the webAPI

auth\views.py:
- added 'queryset' and 'serializer_class' to the 'UserDetail' view.