Encoder: Mp3Encoder add xingmux in the pipe in order to fix duration

From Gstreamer manual :
"xingmux adds a Xing header to MP3 files. This contains information about the duration and size of the file and a seek table and is very useful for getting an almost correct duration and better seeking on VBR MP3 files."
Merge branch 'dev' of https://github.com/yomguy/TimeSide into dev