Initial refactor of profile support for Django 1.7

Following on from the discussion of profile support in #987.

Tasks:

- [ ] refactor signal handling in accounts/models.py so as not to call get_user_model or get_profile_model prematurely.
- [ ] replace all calls to Django's get_model to the wrapper function in mezzanine/utils/models.py which, as in Django 1.7, raises an exception if the model is not found
- [ ] replace calls to ``get_profile()`` with a new Mezzanine-specific function ``get_profile_for_user()``
- [ ] rename ``AUTH_PROFILE_MODULE`` to ``ACCOUNTS_PROFILE_MODEL`` with deprecation warnings
- [ ] tests for all of the above

Let me know if you think this is heading in the right direction Steve.