Options for partial serialization

Hey - me again!

There are cases where you only want to serialize some fields - not all of them, for example when doing a PUT to your API when you only want to modify 1-2 fields, rather than the whole object.

I'm just wondering what the best way of doing that would be.

Perhaps adding a version of `-toJSONString` to `JSONModel.h` which has an `NSArray` of the properties you want to include?

What are your thoughts on this? If you have any other ideas, let me know. Once we've got a way we like, I'll pop a pull request in.

Edit: also begs the question of whether partial deserialization would ever be useful, though I suspect perhaps not.