Set HTMLTemplateElement as a window property
Set HTMLTemplateElement as a window property

Rather than as a global variable, which seems to have troubles with strict mode in a few browsers.

Closes #496
webcomponents/polyfills,jre-g/webcomponentsjs,webcomponents/polyfills,jre-g/webcomponentsjs,webcomponents/polyfills,jre-g/webcomponentsjs