Pygit2 0.21.0 uses cffi

Changes: Now using cffi Upgraded to libgit2 latest and greatest v0.21.0 And much more… Thanks to Carlos Martín Nieto, Michael Jones, Ian P. McCullough, Jack O’Connor, Thomas Kluyver, Daniel Bruce and Jun Omae. Release partially sponsored by Bors LTD Resources: Download from https://pypi.python.org/pypi/pygit2 Documentation at http://www.pygit2.org/ Join us at https://github.com/libgit2/pygit2 Enjoy!