agent: logging HTTP API

HTTP endpoints and tests for the logging api.

-jeff
Here's what I added.

	// logs.Notify does not write to blocked channels, so the channel has to be
	// buffered.  The capacity is chosen so that a burst of log lines won't
	// immediately result in a loss of data during large surge of incoming log
	// lines.
	logLinec := make(chan string, LogBufferSize / 10)