invitation.views.invite requires remaining_invitations > 0 to send invitation.
invitation.views.invite requires remaining_invitations > 0 to send invitation.

Fixes issue #6.

adieu/django-invitation,jannon/django-invitation,jannon/django-invitation,mgaitan/django-invitation,noderabbit-team/django-invitation,lubosz/django-invitation,arctelix/django-invitation,mgaitan/django-invitation,deschler/django-invitation,arctelix/django-invitation,ruiwen/django-invitation,mentionllc/django-invitation