Implement Pull Requests API

Hey @jpsullivan I'm going to finish this off as it's something I want to start using (and also the oldest PR).

Supersedes #173 

Implements http://developer.github.com/v3/pulls/ 

- [x] tidy up outstanding feedback around documentation
- [x] integration test - create a pull request
- [x] integration test - update a pull request
- [x] integration test - close a pull request
- [x] integration test - merge a pull request
- [x] coding style
- [x] some more documentation :heart_decoration: 
- [x] bugfix - PR commits have author details - use different type to encapsulate this info