Update vote and unvote view to handle new model implementation. Fix #10

number_of_votes is a field and it updated by the views automaticaly.

Take a look here:
https://docs.djangoproject.com/en/1.6/ref/models/instances/#updating-attributes-based-on-existing-fields
regarding the race condition. No need to mark the view as atomic.
Add link to registration from login template. Fix #12