Add shortcut for `GET /feeds`

Usage like below:
```objc
RACSignal * listFeeds = [authenticatedClient listFeeds];
      [listFeeds subscribeNext:^(OCTFeed *feed) {
        NSLog(@"timeline url: %@", [feed.timelineURL absoluteString]);
        NSLog(@"user url: %@", [feed.userURL absoluteString]);
        NSLog(@"current user url: %@", [feed.currentUserURL absoluteString]);
        NSLog(@"current user public url: %@", [feed.currentUserPublicURL absoluteString]);
        NSLog(@"current user actor url: %@", [feed.currentUserActorURL absoluteString]);
        NSLog(@"current user org url: %@", [feed.currentUserOrgURL absoluteString]);
      }];
```

:P