Allow custom creation of response object

[pnstickne]

Per #1591

A fairly-generic method of intercepting request creation and specifying custom request objects (and configurations therein). Works for XHR / Image / Audio as well as "fine tuning" the URL or whatnot.

Usage for custom XHR headers looks like:

loader.onCreatingRequest.add(createRequest, this);

function createRequest (loader, r)
{
    if (r.requestType === 'xhr' && shouldCustomizeRequestForUrl(r.url))
    {
        var xhr = loader.createXMLHttpRequest(r.url);
        // any relevant modification to XHR / request can be done here
        // (including providing a fake proxy object or using a different URL entirely)
        // but there are some restrictions and the xhr.responseType will be overridden
        xhr.setRequestHeader('custom-header', 'value');
        r.requestObject = xhr;
    }
}

[pnstickne]

Grr. "npm install failed" for Travis CI - "no 'build' task"

[photonstorm]

How safe is this to merge this close to release? At least I've extensively tested the current 2.3 Loader through several large client projects, but I can see lots of underlying structure changes here.

[pnstickne]

I would say 'fairly safe'. Most of the structure changes come from unifying XDR+XHR code (of which I have sadly not tested for XDR, but XHR works as expected).

The building (and extraction of) the request configuration doesn't really affect the structure (excluding the basic assignment into and out of); any new semantic differences should only be observed if adding an onCreatingRequest handler and altering the supplied values.

Some of the image/audio callback code is updated for consistency and I have not run into any regression cases.

[photonstorm]

I'm afraid I'm going to leave this one be. I'm happy with how the Loader currently works, and will leave features like this for Lazer.