fix some memleaks; time_start, time_limit for yaafe-io; pickling for Dataflow; misc

I apologize that you may find it a little bit hard to review the PR as too many files are changed in https://github.com/mckelvin/Yaafe/commit/a4c37503d0ecd84d72b4a2ecc2ab9da24bf6c667 and 
https://github.com/mckelvin/Yaafe/commit/4ba7c408141da8ae9773e2eca304330115cd2cdb, may be commit-by-commit review is a better choice. I'm not sure whether all the commits are welcomed, and if not so, please comment in the thread without any hesitating.

here are the main changes:
- fix memleaks in:
  + Smarc
  + MP3FileReader
  + freeOutputFormatDescription
- support `time_start`, `time_limit` for `MP3FileReader` and `AudioFileReader` so that we can extract features in a specific time range.
- src_cpp: DataFlow: support `stringify`, `loads`
- src_python: Dataflow: support pickling

this PR should also fix #3