app: starting to build LogListener

A LogListener will provide a channel that will be used to broadcast
messages to the FollowLogHandler.

Related to #194.
app: add Close method to LogListener

This will close the listener, closing the channel to stop anyone ranging
over it, and setting the state to closed, so Log method won't send any
new logs in the channel.

Related to #194.
AUTHORS, CONTRIBUTORS: add Gary Burd