JSON loader changes + SPINE update

JSON Loader Changes.

JSON loader started working wonky after a Chrome Android update, I have my project saved on my home screen and it would not load any JSON files at all. I tested in several devices and the update seemed to break it. Curious thing is that in the browser the behaviour is not 100% reproducible, only when pages are saved to the home screen.

After some investigation, it seems like the onload event in XMLHttpRequest is broken somehow. I found a couple fixes:

- Adding an onprogress event to the request.
- Using the more standard onreadystatechange event

I decided to implement the 'onreadystatechange' option because as MDN states (https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest):
"onreadystatechange as a property of the XMLHttpRequest instance is supported in all browsers. Since then, a number of additional event handlers were implemented in various browsers (onload, onerror, onprogress, etc.)."

The more standard version should be more robust. I made the changes while keeping the XDomainRequest functionality untouched.
   
   
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SPINE Update.

Updated to spine's latest runtime that supports animations exported with spine 2.0+

At this stage, this implementation only support region attachments. I've also added support for IKConstraints and per bone axis flipping. I'll update the other types of attachments as needed.

It implements the concept of "auto update" which is the default behaviour, but it has property to disable it and update the spine state manually, this allows more control over how the animation updates.
Also, implements automatic atlas loading and supports multipage atlases as per spine's specification.