Remove TODO about supporting multiple documents.
For now, let's just support a single document.

Multiple document support is not trivial.   In addition to specifying a document when
creating a widget instance (previously done via new MyWidget({ownerDocument: ...}),
there's document.register() itself, which registers a custom tag with a certain
document.

In the future, multi-document support is needed, maybe it's a question of leveraging
AMD to load multiple instances of the register.js module, each with a different document
specified through module.config().doc.
remove inert code; params not available in buildRendering()
fix typo, use this[foo] to test type of foo property
fixes for latest widget system
improving test