Support solid gray frames when decoding Lagarith.

Fixes bug 431.
doc/muxers: remove outdated comment about live flags for the segment muxer

The live flags is currently unused, but still kept for backward
compatibility reasons (and in case it turns to be useful again for
whatever reason).
lavf/segment: provide link to HTML M3U8 draft, also fix mentioned version in docs
lavf/aiffenc: ID3 tags support

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>