Logging api

Reviewers:  @bernerdschaefer, @peterbourgon 

First pass at the internals of the logging API.  This is two layers thick.  There is a RingBuffer for log messages, and on top of this is a data structure that implements storage for and operations for each container.  (The RingBuffer probably should have been a separate submit.)

The next piece to be worked on will be the HTTP logging interface.
`mux.Get`: https://github.com/soundcloud/harpoon/blob/master/doc/agent-api-v0.md#get-containersidloghistory10