Simple key-value store

The purpose of `barge-store` is to make it easy to test barge with [jepsen](http://github.com/aphyr/jepsen). By exposing a simple put/get interface we can implement a simple client that will be used by jepsen to check linearizability of barge. 

Main program is now `barge-store.jar` (replacing `barge-http.jar`) which launches server in the background. Server can be stopped by `POST`ing empty string to URI `/control/shutdown`. Store is available under namespace `/store` and is dead simple: 

* Use `PUT /store/my/key` with arbitrary byte input to store data. This must be sent to current leader and waits for Raft protocol synchronisation before returning to client,
* Use `GET /store/my/key` to retrieve stored data or a 404 error. Data is read from current state machine so reading from non-Leader might provide stale data.