Updated Astroid API + web2py modules in PYTHONPATH

This version uses makes pylint-web2py work on latest Pylint with changed Astroid API.
Also it finds web2py module directories and adds them to PYTHONPATH, decreasing number of Pylint complaints about undefined objects.

It requires the latest version of Astroid from Mercurial, because release version had a bug causing modules to have empty path. So module couldn't determine if it belongs to web2py.