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`
@dustinlarimer I meant that Id prefer a comment in words rather than a link, to know what the function of 179-189 is. I think `doc block` is a plugin I have which creates multi line comments! Sorry for the confusion :)