Image cache

This is a cleaned up replacement of zags image cache PR (https://github.com/xbmc/Addon-Frontend/pull/4). It also adds a real cache that downloads thumbs from remote repo and caches them locally. In addition I added a script that checks if a addon has a thumbnail and if not returns a dummy thumbnail. I also added a function that automatically resizes and caches images - so resized thumbnails can be created on the fly and get cached.