authentication: allow all transport modes of access token in OAuth2Authentication

RFC6750 describe three transport modes for access tokens when accessing a
protected resource:
- Auhthorization header with the Bearer authentication type
- form-encoded body parameter
- URI query parameter

This patch add support for last two transport modes.
Merge pull request #1469 from entrouvert/master

authentication: allow all transport modes of access token in OAuth2Authentication
Bumped tests against Django from 1.7a2 to 1.7b1
Fixed encoding parameter in QueryDict
Merge pull request #1483 from dn0/querydict-bug

Fixed encoding parameter in QueryDict
Aligned the django-filter version with travis builds.