Introduction of Stateful#observe(), refs #178, fixes #182.

Also replaces delite/Invalidating with decor/Invalidating, which has a slightly
different API.
Fixes related to Stateful#observable() changes, refs #182.
refactor FormValueWidget test
jshint fixes
added doctype to polymer func test, fixes #225.
convert CssState to use observe() rather than watch(), refs #182.
convert template.js to use observe() rather than watch(), refs #182.
Remove delite/Stateful and just use decor/Stateful, refs #182.

Note that if a widget has a tabIndex setter to move tabIndex to another node,
the callback to observe() needs to use this._get("tabIndex") to find out the
new value of the tabIndex property.  Presumably there are still lingering issues such
as that templates with {{tabIndex}} don't work correctly.
fix outdated doc