Pygit2 0.21.1 and a new skin for the documentation

Changes: Documentation: use the read-the-docs theme New Repository.state_cleanup() New Index.conflicts New checkout option to define the target directory And more… Thanks to Carlos Martín Nieto, Vlad Temian and Jasper Lievisse Adriaanse. 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!