Installation should not require Cython

pysam already follows the recommendation by Cython developers not to
require Cython to be installed and to instead distribute the compiled C
files, as recommended here:
http://docs.cython.org/src/reference/compilation.html#distributing-cython-modules

Currently, since an install_requires directive containing Cython is passed
to setup(), Cython will always be installed when running 'python setup.py
install' or 'pip install pysam'. This commit removes 'install_requires'.

In fact, the code as it was does not even work anyway: When running `python setup.py install`, Cython is installed *after* pysam itself. It should probably have been `setup_requires` or so.