Hoodoo::Cache (Trello import)

[pond]

Uses the Transient Store backend once created (see #105) assuming a single shared instance of that store. Knows when someone creates, updates, deletes a resource and thus knows how it's represented for a simple show, or a simple list etc., or when it should invalidate such thing. Idea is to avoid in- or inter-resource calls by having the result already in the cache.

Depends upon #105.

[Andrew Hodgkinson]