agent: /containers emits server-sent events

This is the implementation of #14 for harpoon-agent.
doc/api: update spec for eventsource
agent: drop EventBody from ContainerEvent
doc/api: GET /containers only publishes one event-type, an array
agent: use single /containers event type
scheduler: remove all ContainerEvent references (compiles, doesn't test)
scheduler: re-implement mockAgent containerEvents
scheduler: fix tests
scheduler: agent_test: eventsource.Encode already Flushes
scheduler: use eventsource.Handler in mockAgent
scheduler: remoteAgent *must* send events to listener
Merge pull request #17 from soundcloud/agent-eventsource

Use server-sent events
Simplified messages as per PB's review request.  Removed extraneous gaurd clause.
Merge branch 'master' of github.com:soundcloud/harpoon into logging-api
Merge conflicts fixed.