fallback for browsers that don't support domElement.outerHTML

If you look at the element.outerHTML page below, `outer.outerHTML` is available in most browsers, just not Firefox versions < 11. Even old IE has it. 

Also let me know if I can improve the test. I tried deleting the `outerHTML` property from the element, but that raised an exception. I tried also stubbing `XMLSerializer` by redefining it on `window`, which also threw an exception.

I know that the string I provide in the test is not **really** a DOM element, but I'm stuck on how to stub this out in a better way. Happy for suggestions.

Resources:

http://stackoverflow.com/questions/1700870/how-do-i-do-outerhtml-in-firefox
https://developer.mozilla.org/en-US/docs/XMLSerializer
https://developer.mozilla.org/en-US/docs/DOM/element.outerHTML

fixes emberjs/ember.js#1952