2.0: reduced xhr transport

* removes oldIE tricks
* use of `onload` and `onerror`, not `onreadystatechange`, which makes the whole `isLocal` logic unnecessary
* always get `responseText`, never `responseXML`, just use the internal converter when needs be (removes a lot of controls regarding how (in)valid `responseXML` actually is)

One unit test changed but for the better: `throws` option will now have ajax throw in an event loop which is awesome for the use case (throw exceptions when executing scripts in domManip).

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