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.
Patch supposedly speeding up completion from SublimeText2 module.

Should be solution for #44.

Original commit message:
 - added a lot more caching to Rope
 - added case-insensitive completion
Testing the patch.