Improve bootstrapping python code

[Mathieu Virbel]

The current code generated by Cython have few issues:
* cannot use exec() inside it
* don't initialize C-API thread support (PyEval_InitThreads is not called, then further extension like ffmpeg will not work)
* using import <yourmainfile> cause further import in thread to be stuck at a new import
* the import broke the __name__ == __main__

On kivy/android version, based on pgs4a, i've reimplemented the bootstraping code. It's now written in C, and resolve all issues before. It's available at http://bazaar.launchpad.net/~tito-bankiz/pgs4a/kivy/revision/132

You might be interested to adapt it for pgs4a :)