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.
Please configure your toolchain to run `go vet` and `golint` during your build process. They would catch the error here, which is that LogSet should have a comment (since it's an exported symbol).