Add tests for new class derived from httplib2.Http

This should expose the same functionality and the same interface, but will back off on 503, 502 and 403.
Implementation of class derived from httplib2.Http

This exposes the same functionality and the same interface, but backs off on 503, 502 and 403.

Instances of this (rather than httplib2.Http) will be injected into the Google API client.
Replace instance of Http with one of HttpWithBackoff
Use Httpwithbackoff with gapy in realtime and trending
Merge pull request #22 from alphagov/ga-requests-with-backoff

Ga requests with backoff