WIK Implemented pull request review comments API

*SimpleJson* can't deserialize fields starting with an underscore (```_links``` for example). Is this the expected behavior? In the case of a review comment it's not a big issue as the three links are already contained in the object.

For the ```Create``` [method](http://developer.github.com/v3/pulls/comments/#create-a-comment) should I also ensure that the ```HttpStatus``` is ```201```?

I didn't write any integration tests as it would require at least two users. But I tested all the methods with two of my users. In order to have integration tests we would need to create a "static" test repository on Github where we could run methods knowing it would always return the same results.

Am I supposed to do anything with [Custom media types](http://developer.github.com/v3/pulls/comments/#custom-media-types)?

```number``` is implemented as Int32 currently, should we use Int64 instead (ie Facebook API)?

issue https://github.com/octokit/octokit.net/issues/136