adapt code, test and doc to latest dstore
fix jshint in test
Make appendChild() and insertBefore() work on this.containerNode
rather than widget root node (unless this.containerNode is undefined).

But this only applies after widget creation finishes, because buildRendering()
needs access to the real appendChild() method.

Refs #168.
Move everything related to containerNode to the Container subclass.
This means that any widget that contains content (ex: Button) should extend Container
rather than Widget.
For declarative instantiation, move initial children into this.containerNode too.
make doc explicit about Container being for external children