Adds SINGLE_ACCESS_TOKEN constant (default is False)
Adds failing test for new feature
Implements 'single access token' feature
Updates documentation to reflect new constant
Single Access Token featured applied to password grant as well
Added support for swappable models (like Django 1.5 User model). However, swappable models are not documented, so consider it experimental!
Django 1.5 is minimal version due to swappable models.
Merge with johanmeiring/master
Provide grant, old access token or old refresh_token to create_access_token and create_refresh_token to allow new tokens to be based on old ones (for example, to copy properties).