Features/issue 137

This adds a logger attribute to the LogfileInput to resolve https://github.com/mozilla-services/heka/issues/137

By default the logger will be set to the filename that is being logged.

I've also removed the various maps which were keyed on logfile name and switched them to use just the value type of the map.  

That includes seek_map, ident_map, discover and fds in the FileMonitor