Cloning repositories

Since cloning is now supported in libgit2, I thought it would be useful to support it in pygit2.

This pull request adds initial support to cloning. I intentionally skipped the callbacks due to the docs for libgit2 mentioning performance issues with them.

I need some help with the tests, though. I'm very new to the pygit2 codebase, which means I'm still looking for things that might be easy to find for core committers.

If any of you could help me in finishing the test suite included with this pull request, I'd be more than happy to improve on it.

If I need to submit some docs with it as well, please let me know.