Environment is now better controlled

* Python interpreter is now generated with an 'exec' wrapper
* Develop recipe now uses a self generated python interpreter for the build,
  which allows us to develop locally packages such as 'xbob.extension'. This
  also avoids the directory trashing by distribute/setuptools
* The environment wrapping is now separated into a class that can be re-used
  all over the place
* Each interpreter/script generator now uses it so to assure that packages that
  will be downloaded and compiled under its umbrella do get the proper/expected
  environment