Add DiffStats

[carlosmn]

This wraps git_diff_stats and can be retrieved through a Diff. It
includes a formatting method.

I wasn't quite sure what to call the formatting function. It's called _to_buf() in C but the type is irrelevant in python, and something like to_string() didn't seem to fit as it takes options.

[carlosmn]

Oh, that python 3, I seem to forget how to create strings each time.