Implement HLS Live Streaming

[nschlia]

Add capability to create an HTTP Live Stream (also known as HLS).

https://en.wikipedia.org/wiki/HTTP_Live_Streaming

https://tools.ietf.org/html/draft-pantos-http-live-streaming-19

[nschlia]

HLS now working in beta phase.

Still to todo:

• If a file has not yet been transcoded fully to cache, and e.g. segment 50 is directly accessed, transcoding should start at that segment (similar to frame sets where frame no. 50 would directly be processed when required).
• Segmentation should be made complete, e.g., audio and video must be cut off at the same PTS and start with the same PTS at the next segment. Currently the last segment may contain audio frames that belong to the next segment or the other way round, the next segment may contain audio frames of the last. Does not seem to bother playback, though, but this is not correct.

[nschlia]

• HLS: Introducing direct segment access. If e.g. segment no. 333 is opened, ffmpegfs will seek to this position and start transcoding the segment immediately. Please note that this feature is still experimental and may produce unexpected results.

[nschlia]

Works now :)