Add ability to check if resource is cached

Clients of the API likely do not want to cache all resources
that trigger webhooks. The new method Query.is_cached can be
used to check if a resource is cached before re-fetching.
Merge branch 'master' of github.com:gadventures/gapipy
Merge branch 'master' of github.com:gadventures/gapipy

Conflicts:
	HISTORY.rst
	gapipy/__init__.py