Added XML Comment, short-form closing support

Added support for XML Comment, and short-form closing tags. Also added
ability to get the XML namespace prefix on an element, namespace URI,
and local tag name. Short-form change should take care of issue #48.

Majority of the diff set is formatting change. As a general practice, I use the format capability of the Dart editor to reformat code. This causes lots of white-space and line-break differences