Generate bindings for HTMLTemplateElement.
Generate bindings for HTMLPreElement.
Generate bindings for HTMLLegendElement.
Generate bindings for HTMLLabelElement.
Generate bindings for HTMLTrackElement.
 Generate bindings for HTMLMediaElement.
Generate bindings for HTMLAudioElement, HTMLVideoElement.
Add testcases.
auto merge of #837 : saneyuki/servo/webidl, r=jdm