Add Blame

This implements the current API for blame that @ben has been working on in libgit2#1317.

The current implementation is pretty simplisticand rather low-level:

`Rugged::Blame` is a simple wrapper around `git_blame`. Individual hunks can be accessed through `Rugged::Blame#[]` (for index based access) and `Rugged::Blame#for_line` (for line based access).

Each Blame Hunk itself is exposed as a Ruby Hash, containing the same information as the `git_blame_hunk` struct in libgit2.