Allow logging to be toggled for addon

Hope this is OK, as I'd just done a pull request, thought I'd do another - this is to support toggling logging on and off at the addon level, it helps prevent flooding the log with messages for the case where a user is trying to debug something completely unrelated to this addon.  Discussion on the following thread:

http://forum.kodi.tv/showthread.php?tid=196442

Thanks, Rob