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`
If someone manages to pollute the readKey, they could send an unescaped value via XHR... I don't think that's an actual problem, but it may be safer/better to change this to:
```
params.api_key = this.client.readKey;
urlQueryString  += '?' + getQueryString.call(this, params);
```