Add HLS support for Twitch/Justin

This pull requests adds HLS support for Twitch's channels, and drops the deprecated RTMP support.

It also changes the HLS fetcher to automatically determine baseurl, instead of each service having to pass it in. The idea is that relative URLs in the playlist files are relative to the document that references them, i.e. the m3u file. Note that at least Twitch requires query parameters to be kept as well.

This has not been thoroughly verified, I've only tested against twitch. I will try to do some more verification, and may update the branch if I encounter issues with other services. In particular, I'll try to verify that Justin also works :-).