Listen on control socket for daemon control

Puts together a similar model to that used by Tmux: namely a daemonized backend, but with the same program able to act as a client in order to send it commands. Implementation uses Golang RPC over HTTP with a separate Unix control socket.

``` bash
$ heroku-agent &
[1] 67254

$ heroku-agent state
Up: 4m9.356207271s

$ heroku-agent clear
Cleared all stores
```