XMLHttpRequest#response

Implement XMLHttpRequest#response and #responseType for some more advanced usages like responding with an ArrayBuffer.

I tried to implement http://www.w3.org/TR/XMLHttpRequest/#the-responsetype-attribute and http://www.w3.org/TR/XMLHttpRequest/#the-response-attribute as closely as possible.

We could implement it even better, if we used getters and setters (https://developer.mozilla.org/en-US/docs/JavaScript/Guide/Working_with_Objects#Defining_getters_and_setters), that are available since Javascript 1.5, but they are not supported in IE<9. Is this incompatibility a problem for this project?