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. 
@joshaber I was following the convention as [here](https://github.com/JeffScaturro/octokit.objc/blob/PullRequestsOnRepo/OctoKit/OCTClient%2BRepositories.h#L87). You can combine them into an array as you receive them, i.e. I specifically did (I needed them in another array to attach other data): 
```
[[[[self.client fetchPullRequestsForRepositoryWithName:@"repoName" owner:@"repoOwner"]
       deliverOn:RACScheduler.mainThreadScheduler] collect] subscribeNext:^(NSArray *pullRequests) {
        self.openPullRequests = nil;
        self.openPullRequests = [[NSMutableArray alloc] init];
        for (OCTIssue *issue in pullRequests) {
            NSMutableArray *array = [[NSMutableArray alloc] initWithObjects:issue, nil];
            [self.openPullRequests addObject:array];
        }
    }
```

I did forget to change the documentation class though.