[Cython] Cython on GitHub

All of of Cython serving infrastructure, save the jenkins buildbot, has been moved to GitHub. (Documentation is served via readthedocs.org, and tarballs served from pypi, built from GitHub head). Each trac ticket has been migrated to a github issue. I have updated/redirected what links I can. As for backups, I created https://github.com/cython/cython-issues/tree/master to store the non-repository data, which will be periodically updated. Note that the original trac data (minus session/user info) is there as well. I will also be locally cloning all the repositories in the cython organization, and would recommend at least one other person do the same. - Robert