Remove emitters from Store

[ricejasonf]

Right now Stores have the responsibility of emitting changes to listeners via the StoreEmitter. When a message is given to a store, it would probably be better to have it simply indicate whether that message should be propagated further upstream/downstream and let the Context pass it on.

This would make pub/sub requirements (if any) completely up to the Consumers, which eliminates the need to create custom StoreEmitters that would probably have logic specific to a given Consumer anyways.

[ricejasonf]

e1dec1f