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`
@jshirley @Dokko1230 Our plan for now is to log a message (latest commit), but in the future perhaps we can explore something clever like constructing and submitting a form w/ the event data? A message should suffice for now.