Extend Store (but not tokens) with new ops

CRUD for diffs and notifications.
Expose new store operations through tokens
Merge remote-tracking branch 'origin/master' into wip-extend-store-api

Conflicts:
	lib/Retcon/Store/Memory.hs
	lib/Retcon/Store/PostgreSQL.hs
Merge pull request #50 from anchor/wip-extend-store-api

Extend API for stores and tokens