Refreshing access tokens

- Added access token expiry check
- Refactored models to use expiresMixin
- expiresMixin now uses model updated_at date instead of created_at
- Modified tests to correctly set model updated_at time
- Renamed config parameter for consistency with other similar parameters
Change /authorized to return 404 for expired tokens
Updated device token refresh method to match latest spec
Renamed module
Merge branch 'develop' into refresh-tokens
Return error if user declines to pair client with their account.
Refactor to simplify test code.
Merge branch 'develop' into refresh-tokens
Changed /token error responses to match CPA spec
Removed unused code
Corrected error callback in deleteAccessToken()
Merge pull request #70 from ebu/refresh-tokens

Refreshing access tokens