Updates for GitHub issues #3 and #4

Closes #3 
Closes #4 

Also added in signal names for debug purposes
Yep, you're right.  I feel a bit like a moron. So, something like this...?

```
...
return [[RACSignal createSignal:^(id<RACSubscriber> subscriber) {
        __block HKSampleQuery *qry = [self rac_executeSampleQueryWithSampleOfType:sampleType
                                           predicate:predicate
                                               limit:limit
                                     sortDescriptors:sortDescriptors
                                          completion:^(HKSampleQuery *query, NSArray *results, NSError *error) {
                                              if (!error) {
                                                  [subscriber sendNext:RACTuplePack(query, results)];
                                                  [subscriber sendCompleted];
                                              }
                                              else {
                                                  [subscriber sendError:error];
                                              }
                                          }];
        [self executeQuery:qry];
        
        return [RACDisposable disposableWithBlock: ^{
            [self stopQuery:qry];
        }];
    }]
...
```