Separate the events subsystem

This separates the events subsystem from the deprecated Server object,
and moves it to its own package, which is cleanly installed into
the engine as a builtin.

The events subsystem exposes the following commands on the engine:

* ```events [FROM]``` listens for all matching events and streams them
to stdout

* ```logevent``` publishes a new event to all listeners (events are not
persisted). Events are duplicated across all active listeners.

* ```events_info``` returns information about the events subsystem.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)