Added support for HLS streaming and restored live tv

Added HLS streamer class for live and on-demand. HLS streamer requires
ffmpeg/avconv. Live TV HLS streams must be re-encoded because
pts/dts differences will kill ffmpeg.

Live TV restored using mobile streams.  HD not available in mobile
streams. Some live HD streams are available with --hds-playlist, which
parses Adobe HDS manifests to find stream URLs and converts to HLS playlists.
HD not available for all channels using HDS manifests (only found for
BBC One/Two/Three/News).

Added HLS-related values for --modes similar to flash mode values, e.g.,
hlsbest, hlsvhigh, hlsdefault, etc.  There is now vbetter/hlsvbetter
alias (only picks up SD live tv, same as better/hlsbetter for other types).