StorehausSourceTap

Addresses https://github.com/twitter/storehaus/issues/208

This is an untested first draft that adds some basic cascading wiring.
Sending it out early for any design feedback as well as any missing big picture items.

I am thinking of having the source tap completely done before moving to the sink tap.

- [ ] add source tap
- [ ] add an example MapStore based source
- [ ] add test
- [ ] integration test with scalding + summingbird