Allow custom sources in the Python install

Hi,
This is the first commit for adding custom sources to the python installation.  By now, it only adds the parameters and logic in the main python\install.pp class.

The default behaviour is to install 2.7.3, but now the user can specify his own sources for installing other version (example in install.pp documentation)

DO NOT MERGE because:

Tomorrow, I want to improve the class python\dependency.pp (documentation and parameters) and make a better integration with the main module, I think it can be ready by lunchtime.

If you have time, take a look and feedback-me :)

Cheers,
Luis