Test Python 3.8 (#136) and use Cython if available (#134)

[eriknw]

Python 3.8 failed to install cytoolz b/c the .c files created by Cython were not forward-compatible. This justifies always using Cython if available.

Also, add Cython as an optional dependency via extras_require in setup.py. This should allow pip install "cytoolz[cython]" to do the right thing.

[eriknw]

Rules are as follows (apply first applicable rule):

1. If --no-cython or --without-cython are used, then only build from .*c files.
2. If this is a dev version, then cythonize only the files that have changed.
3. If --cython or --with-cython are used, then force cythonize all files.
4. If no arguments are passed, then force cythonize all files if Cython is available, else build from *.c files.

By forcing cythonization of all files (except in dev) if Cython is available, we avoid the case where the generated *.c files are not forward-compatible.