Add soundcloud autoresponse plugin.

Currently supports tracks/playlists and users. The /resolve endpoint doesn't send any kind of information about what the item is, except missing items in the response (like .title, which is only in track and playlist responses). So I catch the AttributeError there and switch to returning username instead.
Update requirements.txt
Update config.py
Merge pull request #76 from blha303/patch-6

Add soundcloud autoresponse plugin.