Implemented HttpAdapter with guzzle

I'm gonna add some comments to the code, would like for you guys to let me know what you think about it.
This is actually fine, I have to go over the other things first to see if everything is correct though. The caching mechanism makes sure that every bit of metadata returned is cached and it doesn't cache by call but by normalised key. So if you've fetched all the meta-data you won't have to refetch it when just asking for the timestamp or mime-type. The `mimetype` key will just be fetched from cache.