Transcoder: can add a dummy with a profile name

* Need this new function to add a dummy stream with a preset name (no
profile desc is needed).
Transcoder: can add a dummy with a profile name and a subStreamIndex indicated

Complete the API, with keeping symmetry. Now there are:
     * 4 add functions with only a streamIndex indicated
     * 4 add functions with a streamIndex and a subStreamIndex indicated
Transcoder: add doc for enum EProcessMethod
Transcoder: complete constructor
avprocessor: can add dummy video

* If dummy video, need a VideoDesc.
* Warning: can't add a dummy audio, because we need an AudioDesc... Need
to add this information in the confi file. Maybe it will be easier in
python!
avprocessor: setProcessMethod to shortest
Merge pull request #38 from cchampet/dev_Transcoder

Transcoder: complete API