fixing docstrings format.
complete rewrite of views.py's code base to use django-registration0.9's class based views
deleted ``RegistrationFormInvitation``. it's not needed anymore since
user saving is being done in the ``RegistrationView``.
removing DeprecationWarning, changing django.utils.hashcompat for
hashlib.
minor fix in ``RegistrationView``.
removing ``InvitationStat`` and everything about it from models.py.
updating urls to use the new class based views.
removing 'invitation/' from all the urls as they will be included with
a common subpath.
removing ``InvitationStat`` from admin.py
removing templates and templatetags
removing `create_stats_for_existing_users`.
added new `delete_expired_keys` to ``InvitationManager``.  also added
a new command ``cleanupinvitations`` that triggers this method and can
be included as part of the routine site maintenance.
changing `django.conf.urls.defaults` for `django.conf.urls`.
typofix in invitation/urls.py
removing stats from tests and making them 1.5 friendly.
bugfixes in ``RegistrationView``.
updating setup.py
removing some un-used settings.
adding myself as an author.