Change to allow the user to keep iOS-style TV show artwork locally


You just need to drop a ```cover.jpg``` file into your Sickbeard paths (under ```root_dirs```) or the same directory as your file and it will pick them up.  

* *(This is primarily for iOS devices since they only show square covers for TV shows but TVDB only provides portrait-styled images.)*
* *Ideally, this would be a setting where the file could be set arbitrarily to any valid image (i.e. PNG, JPG) I'm happy to add a more thorough version if there is interest in something like that.*