Bumped HTMLCollection interface to latest

Spec:
http://dom.spec.whatwg.org/#interface-htmlcollection

This is a sub-task for #1662.
Added HTMLCollection::create

This is a sub-task for #1662.
Implemented {Document,Element}.getElementsByTagName
Implemented {Document,Element}.getElementsByTagNameNS
Implemented {Document,Element}.getElementsByClassName
Replaced Document::createHTMLDocument in favor of HTMLCollection helpers
Added FIXME stub comments

This is a sub-task for #1662.
Added HTMLCollection content test
auto merge of #1838 : brunoabinader/servo/htmlcollection-live, r=jdm