PromiseKit is now supported when cache policy is CJAPIRequestReturnCacheDataThenLoad

Now you can get a clean interface like this:

```objective-c
[request perform]
.then(^(NSDictionary *cachedResponse, Promise *nextRequest) {
  //Cached or fresh data, do stuff with it
  [self doStuffWithData:cachedResponse];
  
  if (nextRequest) {
   //The presence of nextRequest indicates this was a cached response,
   //return it so that the next "then" is called when the fresh request completes
    return nextRequest;
  }
})
.then(^(NSDictionary *freshResponse){
  //Fresh data, do stuff with it
  [self doStuffWithData:freshResponse];
});
```