Enhance selectors and templating when dealing with js/DocumentFragment

[arnaudbos]

Summary:

• Improve template/->node-like and template/->document-fragment by extending the PElement protocol with js/Comment for template from remote locations;
• Ensure sel and sel1 still work when executing className and tagName selectors on a DocumentFragment by using the querySelector and querySelectorAll APIs;
• Add corresponding tests.

Thanks for the great library!