Add support for signed commits

[mainrs]

Edit: I did a prototype using gpgme, but that only works for Unix systems. I couldn't find an alternative way to sign commits on Windows. For that we'd have to:

1. Create the commit buffer instead of committing directly (create_commit_buffer).
2. Use OpenPGP to sign the contents of the commit_buffer.
3. Call commit_signed on the repository instance.