Setting a Callback for an HttpResponse
<p>I'm consuming a 3rd party API (REST JSON-based).  In it they require a callback url so I can grab a token that's returned.</p>

<p>So how does this work?  If I give them a url, how is it hooking back up to my callback method?  How do I specify the method or where it's at once the response gets back... how do you wire all this?</p>

<p>So in the header of my <code>HttpRequest</code>, I'm adding a custom value of "callbackUrl", "ourCallbackUrlHere" as required by the API.</p>
