Add formula for cython.

And's yes, its on pypi, but it always fails to install (or is missing several programs) with pip/easy_install because its written in C, compiles python to C, is meant to be installed in a standalone configuration, and there for is much better suited for a package manger that well, knows how to change CFLAGS. I know there's aways a little bit of bleed-over: you can `pip install hg` and `easy_install dblatex` too — so if ever there were a candidate for this, its this one.