…o decode

* New attribute: _continueWithGenerator
* New method: continueWithGenerator

To be able to generate data (ie silence or black) when there is no more
data to decode.

* This was an old bug, which occured if we try to generate silence,
after transcoding a real audio stream with a last packet very small.
* Add pyTest to check this execution.
* It could be much beffer to reallocate the given frame of the
generator, and put in it more samples of silence.

Fixed a Travis error:
"The log length has exceeded the limit of 4 MB (this usually means that
the test suite is raising the same exception over and over)."