Add three methods to fetch pull request(s).

**DESCRIPTION**
I was using this framework when writing an iOS app and needing to grab pull requests (open, closed, and occasionally a single one). I could not find any methods that did this, so I added my own. I thought it would be nice to add so others could use as well. 

I needed to get objects of pull requests to do things like display open ones, and get comments on a given pull request (I could push later if that would be nice to have as well), and display them in my app. 

**WHAT WAS CHANGED**
I added three methods: 

1. `(RACSignal *)fetchOpenPullRequestsForRepositoryWithName:(NSString *)name owner:(NSString *)owner;`
  * This method fetches all open pull requests on a repository and returns an OCTIssue.
2. ` (RACSignal *)fetchClosedPullRequestsForRepositoryWithName:(NSString *)name owner:(NSString *)owner;`
  * This method fetches all closed pull requests on a repository and returns an OCTIssue.
3. ` (RACSignal *)fetchSinglePullRequestForRepositoryWithName:(NSString *)name owner:(NSString *)owner number:(NSInteger)number;`
  * This method fetches a single pull request on a repository and returns an OCTPullRequest.

@jspahrsummers - If there is any convention I did not follow or anything you would prefer to be done differently let me know. 
Thinking about it now, you are correct. I was dealing with a repository where we do not use issues, but strictly pull requests (so did not catch this). 

The reason why I went issues is because if you want to comment on any pull request is you have to have the issue number. After all, a pull request is just an issue with code. 

After seeing the [link relations](https://developer.github.com/v3/pulls/#link-relations) it would still be possible for a user to do things such as commenting if this returned an `OCTPullRequest` by simply using `OCTPullRequest.issue`.

I will get that changed up shortly. 

@joshaber