Sort param on Pull Request -> getAll method

Hey @mikedeboer, it seems that `node-github` doesn't support [`sort`](http://developer.github.com/v3/pulls/#list-pull-requests) parameter on `Pull Request -> getAll` method. I need to use this parameter on [node-gh](https://github.com/node-gh/gh).

Do you think it's a good idea to add it on node-github API?
I can submit a patch if you point me on the correct direction.

Thanks.