Referral Code API's

@aaustin 

Will add docs later.
I'm worried about this being an asynchronous call. Imagine being the developer, and you build a form that let's a user type in a referral code. You have no validation if the code redeemed successfully. This is going to get messy. We'll need to add a callback here, that will update the credits for a user on success, and notify the developer that the code was successful or unsuccessfully redeemd