Fix optional Cython requirement

Require Cython>=0.20. Older versions wont work anymore. Use pip's de-facto standard to specify requirements ('requirements.txt') to avoid requirements getting out of sync in various places. Use it in tox.ini and .travis.yml.


Add 'fast' extra_require. Allows other projects to demand a Cython-compiled lxml without having to
add Cython to their own list of dependencies. By specifying extra_require, other projects just have to add this to their setup.py:

    install_requires = ["lxml[fast]"],

They don't need to track the correct version anymore. And they don't need to fix should lxml move to Shed_Skin or another Py-to-C compiler.

See http://pythonhosted.org/setuptools/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies