Add possibility to read XHR headers in events

[elbebass]

We use this for transfering additional information like active menu id, which isn't part of the loaded content.

[thybag]

Hello,

I've just pushed an initial implementation of this feature to the develop branch: https://github.com/thybag/PJAX-Standalone/tree/develop

In the develop code, header data should now be returned as an object accessible from PJAX-standalone events in event.data.headers.

[elbebass]

Thanks for this feature.