fix(setup): don't warn re: cython under pypy

When installing Falcon using pypy, don't bother checking for cython -
it's meaningless for pypy.
fix(setup): NameError -> AttributeError
refactor(setup): --cruft, with_cython -> CYTHON
Merge pull request #171 from cabrera/pypy_no_warn_cython

fix(setup): don't warn re: cython under pypy

Fixes issue #167