[XHR2] ArrayBuffer support added

[Anne van Kesteren]

I somehow missed that a request to add back ArrayBuffer support was  
offlist. Since quite a few specifications are using it now and TC 39 has  
shown no progress on developing an alternative I was convinced to add it  
back in. The responseType value is "arraybuffer". This adds a dependency  
to the Typed Array specification developed at Khronos.

http://dev.w3.org/2006/webapi/XMLHttpRequest-2/

--
Anne van Kesteren
http://annevankesteren.nl/