Code to support custom elements w/out Polymer or native custom element support.
Not tested yet.   Known limitations:

- cannot create widgets via document.createElement(); use new MyWidget() instead
- app needs to call parse() to instantiate declarative widgets; it should call parse() after all widget classes have been registered
- app needs to call insertedCallback() manually when programatically created widgets are inserted into the dom
- removedCallback() isn't called when the widget is removed from the DOM
- attributeChangedCallback() not being called; it can be easily added if needed though
get code working (but doesn't do much b/c no template support yet)