Transcoder: call init in process method

* No need to expect the user to call init() before process().
* Note: the init() method can be called several times with no side
effects.
* Update pyTest.
Transcoder: add private method getTotalDurationFromProcessMethod

In order to simplify process method.
Transcoder: refactore process

* Clean indentation.
* Avoid while( 1 ).
* Suppress warning of getTotalDurationFromProcessMethod: add default case in switch.
Transcoder: add private method manageInfinityStreamFromProcessMethod

Can call setProcessMethod before adding the streams.
genericProcessor: clean

* Remove comment.
* No need to call init().
Transcoder: manage FFmpeg log level

* Set it to AV_LOG_QUIET in Transcoder constructor.
* Set it to AV_LOG_DEBUG if setVerbose of Transcoder to true.
Merge pull request #12 from cchampet/move_init_to_process

Transcoder clean