Move to wrapping a standard custom elements polyfill

[pimterry]

Depends on #20, but should be fairly easy once that's out the way. Could continue polyfilling myself, but while that's fine for a PoC, it doesn't seem like the right approach for real usage. Good polyfill: https://github.com/WebReflection/document-register-element

[gilbert]

Just discovered this V1 polyfill, used by polymer, which is only 3.4kb gzipped https://github.com/webcomponents/custom-elements