Refactor request handling for XHR, JSONP and image beacons

* XHR now attempts to use `MSXML2.XMLHTTP.3.0`, `MSXML2.XMLHTTP`, or `Microsoft.XMLHTTP` objects if `XMLHttpRequest` is not supported
* XHR, JSONP and image beacons are all now generally reusable, and may be exposed under utils in a later release, pending some signature (in)decisions for JSONP and beacons
* `uploadEvent` and `sendQuery` functions revert to XHR when URL exceeds `Keen.urlMaxLength`
Maybe console.warn would be appropriate in this case.. If only cross browser support would be good for it...

I'm one for turning it off when XHR isn't supported and the payload goes over the URL length threshold. Wouldn't it be considered malformed data at that point?