Implementation of OAuth with REST API
Fixes to OAuth implementation

Due to the use of django-oauth-toolkit, some classes needed to be changed.
More information can be found at: https://django-oauth-toolkit.readthedocs.org/en/0.5.0/rest-framework/getting_started.html

Basic testing was performed and appears to work correctly with Django REST Framework resource access permissions.
Altered reference for `settings.LANGUAGES`

Removed the direct reference to `myrg_core`.
Alteration to username form field

* `username` now a regex field
 * Regex restricted to \w, ., - in the ASCII range
 * Dropping Unicode as previously envisaged due to problems with future implementation of messaging system
Field validation for username, email addresses

Moved field validation to model definition.
Reintroducing `SessionAuthentication` auth

`rest_framework.authentication.SessionAuthentication` is required for API frontend login
Minor corrections