[web-audio-api] Add normative reference to XHR spec (#36)

> Originally reported on W3C Bugzilla [ISSUE-21527](https://www.w3.org/Bugs/Public/show_bug.cgi?id=21527) Tue, 02 Apr 2013 14:58:44 GMT
> Reported by Olivier Thereaux
> Assigned to 

Per discussion at Audio WG f2f 2013-03-26

The XHR spec should have an entry in the web audio API references table, and we should point all references in the prose to that entry.

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Reply to this email directly or view it on GitHub:
https://github.com/WebAudio/web-audio-api/issues/36