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 single line var declarations always look weird to me, but I see from the code that that is the preferred style so thats cool, maybe a style guide for the repo would be handy. Perhaps you guys could have a link to a styleguide which represents your code style in a CONTRIBUTION.md file?