Add 'get_pymodule' method to 'project'.

This hides some of the 'pycore' functionality and provides a much more
descriptive name.
Update documentation because rope's using GitHub now.
Move get_python_files from pycore into project.

All access of get_python_files are an indirection from project. And the
method itself calls a method that's within the project object. Moving
the method into project cleans this up.