Add new API to post ADNPost objects with a replyTo

Hi, I am in the process of converting my project (which uses Matt Rubin's AppDotNet) to using ADNKit.  I've got most of my app converted over.  I hit a bit of a snag though, in that to post an ADNPost in reply to another, using Matt's AppDotNet I could simply do:

```objective-c
ADNPost *myPost = [[ADNPost alloc] init];
myPost.text = @"Hello World";
// set up other stuff like annotations, file oembeds, etc.
myPost.replyToId = replyToPost.postId;
```

I didn't see anything comparable in ADNKit.  I could use `[[ANKClient sharedClient] createPostWithText:myPost.text inReplyToPost:replyToPost completion:...]` but that doesn't let me add annotations or otherwise customize the ANKPost object, etc.

What would be ideal is something like `[[ANKClient sharedClient] createPost:(ANKPost *)post inReplyToPost:(ANKPost *)replyToPost completion:...]`  Which is exactly what I just created.  (I also threw in `[[ANKClient sharedClient] createPost:(ANKPost *)post inReplyToPostWithID:(NSString *)postID completion:...]` at no extra charge. ;-) )

The only thing I will say of my code is that it works.  In particular I'm sure there's probably a more efficient way of creating the parameters dictionary (rather than copying to an NSMutableDictionary, adding necessary objects and then copying back which is what I did).  Would appreciate some pointers there.