PEP302 based importers for frozen app

This implements the PEP302 based import hooks by extending  sys.meta_path. This new code completely replaces old  iu.py file by using features available in Python.

The main file is ./PyInstaller/loader/pyi_importers.  It contains importers for frozen modules, built-in modules and C-extensions.

By using PEP302 import hooks the frozen apps are able to use the built-in zipimporter available on sys.path_hooks.

http://www.python.org/dev/peps/pep-0302/