[Proposal]Add StoreGroup feature for aggregating state flow

[yonekawa]

Redux has very interest concept.
A single state tree makes it easier to debug or introspect an application.
Redux provides undo/redo state easily because the tree managed all state changes.

Before I tried to port Redux features in SwiftFlux.
However, Redux principles are based on single state tree.
Flux dispatches action to multiple stores. Redux does not need Dispatcher.
#10

I think that SwiftFlux needs to a single state tree utility for developer who needs constraint.