Format Element#tagName correctly when there's a prefix.

format!("{}", atom) yields strings like "Atom('span' type=Inline)", which is
not intended here.
Move build_element_from_tag out of the HTML parser.

This function is not particular to the parser, so should live in the DOM.
auto merge of #3853 : Ms2ger/servo/create, r=Manishearth

This function is not particular to the parser, so should live in the DOM.
auto merge of #3856 : Ms2ger/servo/tagname, r=Manishearth

format!("{}", atom) yields strings like "Atom('span' type=Inline)", which is
not intended here.
Cleanup Element's AttributeHandlers

Remove notify_attribute_{changed,removed} in favor of a unified
notify_content_changed.