Provide outerHTML polyfill

Basic support for the element.outerHTML property does not exist in browsers
older than:

Firefox (Gecko) 11
Chrome 0.2
Internet Explorer 4.0
Opera 7
Safari 1.3

There are other places in Aloha Editor code where element.outerHTML support is
being assumed--where the polyfill should be used instead.  This remains an open
issue.
Refactor comments in core (minor, non-functional changes)
Coalesce <a> fragments during link insertion

Successive anchor elements are generally not merged in Aloha Editor for obvious
reasons, but an exception needs to be made when inserting new links: fragments
of a new link are coalesced whenever possible.
Don't instantiate XMLSerializer in unsupportive browsers