Agent

This is the new agent.  It removes the "on-demand" connecting to each Docker host via docker-py and instead uses a Go based agent (https://github.com/shipyard/shipyard-agent).  The agent pushes containers, images, etc. at a specified interval.  This makes it much easier to scale to many Docker hosts without impacting performance.  This also lays the groundwork for utilizing the agent for more host-level integrations (some to come in the near future are container hooks and host/container metrics).