I-frame playlist support

This adds I-frame playlist support to the m3u8 library, which is required to enable Apple's Trick Play™ mode for fast forward and reverse playback. 

In addition, this adds byte-range support for segments, which is required for I-frame playlists.

See Apple's documentation on HTTP Live Streaming: https://developer.apple.com/library/ios/technotes/tn2288/_index.html