StreamTranscoder: add offset at the beginning of the stream

* Transcoder: can specified an offset when add a StreamTranscoder.
* StreamTranscoder: can set the offset.
* Add documenation about the offset.
Transcoder: add setOutputFps
StreamTranscoder: clean - remove _infiniteProcess

All the politic of transcode is in the Transcoder.
Transcoder: refactoring - switching to dummy into StreamTranscoder

* Switch to a dummy or not, is the responsability of the
StreamTranscoder.
* When set the transcode politic of the Transcoder, prepare each stream
to switch to dummy or note if necessary.
* fix #46
Transcoder & StreamTranscoder: clean

* Remove unused bool _transcodeStream and function isTranscodeStream.
* Suppress spaces.
* Suppress comments.
* Add verbose (symmetry between processTranscode functions).
Transcoder: add doc
Trancoder: refactoring - processFrame
Merge pull request #47 from cchampet/dev_offset

Transcoder: add offset