Makes app compatible with default/custom User model

Changed ForeignKey fields to use string import for
the User model. If no 'AUTH_USER_MODEL' setting is
specified in the settings the default of 'auth.User'
is used.
Adds 'expired_at' field to referral model

Added nullable DateTimeField to allow for
ability to expire a model. No supporting methods included.
Merge pull request #12 from ahmad19526/6_compatible_user_model

compatible user model/ expired_at field