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
- [ ] buffer blame
- [ ] API review
- [ ] XML doc-comments