Use cythonize to build

Use cythonize to build the module. This allows for multithreaded
compilation and makes it easier to use multiple .pyx files.
Get names of files on calculator

Add functionality to retrieve the names of files on the calculator from
a directory listing. For now, an array of strings containing file names
(for vars as well as apps) is returned. Likely, these will be wrapped by
types of some sort in the future.
Update README

Improve the example in the README and show some more of the available
functionality.