docker watcher proposal: identify services by label instead of port and image name

Docker 1.6 added [labels](https://docs.docker.com/userguide/labels-custom-metadata/) for images and containers.

Instead of requiring the same port for every service and exposing the port to the host (#109, #88), read a `com.airbnb.synapse.service-port` label from the image or container, and register the service using that port.

Another option might be to keep specifying the port in the watcher config, but look for a `com.airbnb.synapse.service` label.

Containers without a label would not be registered.

Start by locating the Docker watcher entry point and review the related issues #109 and #88. Resolve whether the label supplies the service port or only identifies the service, then define the selected label behavior for images and containers. Done means labeled services are registered as specified and containers without a label are excluded.

Written by the indexing model from the issue text.