RF: more informative cython error messages

Say when we can't import cython, and when we have the wrong version.
BF: add fake_pyrex hack to MANIFEST.in

We need the `fake_pyrex` directory for setuptools install, but I hadn't
included it in the MANIFEST.in, so setuptools-based installation failed
from a source tarball - and therefore pip.
RF: add bdist_wheel as setuptools command

bdist_wheel is a setuptools extension.
BF: always use fake_pyrex fix for setuptools

I was only using fake_pyrex fix in case where I was forcing setuptools,
not the case where something else (like pip) had already imported
setuptools.