both occur in various states of disconnectedness

- Add interfaces to allow for alternative implementations for Collector, ControlRouter
  and PipeRouter.
- Pass contexts on http handlers to these interfaces.  Although not used by the current
  (local, in-memory) implementations, the idea is this will be used to pass headers to
  implementations which support multitenancy (by, for instance, putting an authenticating
  reverse proxy in form of the app, and then inspecting the headers of the request for
  a used id).

- Add store of pipes in the app
- Add pipe type, handling impedance mismatch, used in app and probe.
- App <-> Probe pipes have their own websockets.
- Add pipe websocket endpoint in app.
- Pipe IDs are strings, lose the request/response IDs, and give the json encoder lowercase field names.
- Add simple golang ws client, for testing.
- Pipe lifecycle plumbing.
- Ref count and timeout both ends of pipes in the app
- Deal with POST /api/pipe/:pid?_method=delete
- Add end-to-end unit test for pipes.
- Add test for timing out pipes.
- Update go-docker client to tomwilkie/go-dockerclient
- Backend work for non-raw ttys
- Close pipes when they close themselves in the probe
- Ensure all http connections are done before returning from client.Stop()