Added NSFW filter to the wallpaper fetcher

% rWallpaper0r.py reacts to parameter --nsfw-filter
+ WallpaperFetcher has a property nsfw_filter and if it is set to True, no over_18 flagged entry is fetched.
+ if more than 25 entries don't match the criteria, an error message is printed
+ if the extension guessing does not work, the url extension is used.
Updated README