Feature request to pass HTTP_ORIGIN with CORS

Chris,

I added the feature. The issue with the response it that you don't have the request object:
http://sysmagazine.com/posts/166539/

So I did the following:
* wrapped the background.js in a method - to avoid global scope (this reduces warnings by ugilfyjs)
* added a **requestIds** array / stack that caches the request data. I allow 10s before the request is GC'd
* added a **onBeforeSendHeaders** handler - to find the request data
* moved the **onErrorOccurred** to **onErrorOccurredHandler** to continue with the same logic and not stack the event handler on every change
* added the logic to change HTTP_ORIGIN to "Origin/Referer/*" in that order
* tested it with my garmin.com.js plugin - works great!! 


valid point, the request should be removed from the ```requestIds``` inside the ```getRequestAndCleanUp``` method then.  

Did you intend to periodically clean out 'expired' requests? I don't see that logic here