Blame it on the Rain

Bind the `git_blame_*` APIs.

![image](https://f.cloud.github.com/assets/39902/1486403/1a9adc60-4723-11e3-8981-72b15d0324d9.png)

- [x] marshalling
- [x] file blame
- [x] XML doc-comments
- [x] ~~buffer blame - what should the signature be?~~ Remove buffer-blame binding until needed
- [x] ~~Should this and Diff go into a `Changes` namespace?~~ (separate PR)
- [x] Demonstrate how to blame a file that has been removed in a previous commit
- [x] `Until`/`Since` naming
- [x] Squash
- [x] 1-based or 0-based line numbers?

Closes #492.