my little proxy

Factored proxy idea out of instrumented branch as its useful in many other context for store extensions. I put an example in the scala doc in the core module and defined proxy interfaces for {Readable,Writable,Mergeabl}Stores. I changed the names a bit from my other branch but I can update that branch after this gets reviewed.

The general idea is that there's a proxy implementation of `Proxied[T]` defined for each core store type `T`. 

Combinators and other custom functionality can now create a new `XXXProxy` for an instance of `XXX`  overriding only what they need. All other method calls are automatically forwarded to the proxied instance