Prior to this commit, there is no mechanism to authorize resources using OAuth2

protocol. In this commit, OAuth2 authorization mechanism is added to

the current authorization mechanism for web API resources.

Services now can follow OAuth2 protocol, send a request with appropriate

authorization header to work with Djblet's resources. External services can

request a token with specify scope through OAuth2 protocol. When using Djblet's

API resource, the scope of a token is checked to make sure that the

external service has the right permission the resource.
This commit also adding unit tests for all current backend authentication:

the base authentication class, the basic authentication via username and

password, the api_token authentication, and the newly added oauth2

authentication backend.

Add test_base_auth_backend to test WebAPIAuthBackend

Add test_basic_auth_backend to test WebAPIBasicAuthBackend

Add test_api_auth_backend to test WebAPITokenAuthBackend

Add test_oauth2_auth_backend to test OAuth2TokenAuthBackend
There should be one more for ResourceOAuth2TokenMixin