Added option to exclude certain folders from scanning

I've had issues with Artwork Downloader scanning folders of non-IMDB media like home movies, web shows or adult movies and sometimes even downloading the wrong artwork for them when names would match up. After some fiddling there didn't seem to be any good solution to limit the scanning process only to a subset of my library. Now I added a new folder exclusion setting to AD to skip all media in certain folders and subfolders, very similar to exclusions in the trakt.tv plugin for XBMC. Right now it's limited to a maximum of 3 exclusion rules.

I have very little experience with Python or scripting for XBMC so I'm probably not aware of any unforseen side effects or possible bugs in my few lines of code. However, it did seem to work just fine in my few test scenarios both on Windows and Raspberry Pi (OpenElec distro).

(relevant posts in the AD forum thread regarding this missing feature: http://forum.xbmc.org/showthread.php?tid=114633&pid=1768736#pid1768736)