[IMP] Add the possibility to define the number of threads used by ffmpeg

[qtheuret]

To improve the performance of video encoding, ffmpeg is able to use multiple cores of the processor (See http://ffmpeg.org/pipermail/ffmpeg-user/2011-July/001735.html)

My pull request allows user to define the number of threads to use for the conversion. It adds a

-threads nb_threads

options to ffmpeg. This option can be set by putting a key 'threads' with a number of threads to use in the Converter.convert() method.