Use Cython.Build.cythonize to cythonize extensions

This is the recommended approach and solves some issues. See:

https://github.com/cython/cython/wiki/enhancements-distutils_preprocessing
Distribute .c sources for cython files as fallback

Since Cython is a non-trivial dependency that may not be available on
every target system it is highly recommended to ship the generated .c
files along with the .pyx sources. See also:

http://docs.cython.org/src/reference/compilation.html#distributing-cython-modules
Fix setuptools issue with cython extensions on python26

When importing setuptools it monkey-patches distutils and replaces the
Extension class. The replaced version converts '.pyx' extensions to '.c'
automatically if it detects that neither Pyrex nor Cython are installed.
However, earlier versions of setuptools do not know about Cython and
check only for the availability of Pyrex.

See also: http://code.activestate.com/lists/python-distutils-sig/17586/