Pasting HTML fix

Tested to work in:
* Firefox Aurora 23 (should work in Firefox 22, which comes out this month-- does *not* work in FF 21, but will "gracefully" fall back to the old broken behaviour).
* Chrome latest stable
* Safari latest stable
* IE 10

No unit tests, since the clipboard is *very* protected by modern browsers. Being able to read/write the clipboard automatically would be a severe vulnerability. As such, the clipboard is only exposed when the user triggers a copy/cut/paste via the browser's own chrome, and only in the actual event is the data exposed.

Should allow copy/paste of arbitrary text/html safely and "intuitively", while still keeping selections/undo-history working correctly.