Sketch of WindowedJoin

DO NOT MERGE YET

closes #208

This is a sketch of a Windowed Join implementation. The idea is to add a Window type to the Platform. In storm, this is probably a Memcache (or similar) store that has a finite in time or size memory.  For storm with a small enough window, it could even be in-memory storage in the storm bolts (if GC does not kill us).

In scalding it is a path that records the key/value stream and does a windowed LookupJoin.

I think this is close to what we want, but looking forward to comments.