connection: Rename "Client" to "Connection"

 * make an attempt to properly document methods and structs for godoc
 * move "Handler" interface to message.go since a Handler deals with
   irc.Message's, not irc.Connection's.

Signed-off-by: Wayne Warren <waynr@sdf.org>
examples: Fix example bots for recent changes.

Signed-off-by: Wayne Warren <waynr@sdf.org>