Fix get_access_token failing on single tracker mode after token refresh (#70).
Merge pull request #71 from Tictrac/bugfix/single-access-token-refresh

Fixes #70. Prevent multiple access tokens from being returned.
#56 - Uncomment an assertion in scope tests. It was commented out whilst adding support for Django 1.6.
Fix serialization of the Client model, when user is not set.