Implement HTMLElement.outerHTML.

Using the spec at
http://domparsing.spec.whatwg.org/#outerhtml

Note that Document does not really have outerHTML or innerHTML
properties in the spec; they are added in domino as handy helpers
to get at the serialization of the complete document.
Add test cases for innerHTML and outerHTML properties.
Merge pull request #18 from cscott/outerHTML

outerHTML support.