Highlight use of permissions alnog with authentication
Add `Unauthenticated` exception.
Add docs for 401 vs 403 responses
Use correct status code
Use correct status code
Use two seperate exceptions - `AuthenticationFailed`, and `NotAuthenticated`

Cleaner seperation of exception and resulting HTTP response.
Should result in more obvious error messages.
Merge with master
Remove OAuth2 from docs
Implementing 401 vs 403 responses
Merged to latest master
WWW-Authenticate responses
Merge branch 'master' into unauthenticated_response

Conflicts:
	docs/api-guide/authentication.md
Merge pull request #416 from tomchristie/unauthenticated_response

Unauthenticated requests - 401 vs 403 responses