Don't apply initialization parameters until after createdCallback() returns.
This means that buildRendering() runs before any user the user defined parameters
are available.

This change is necessary to be forward compatible with browsers that implement
document.register() natively, because in that case programmatic creation (w/out
the syntactic sugar) will be:

var node = document.createElement("dui-slider");
node.max = 10;

Other changes:
- removed runAfterRender() helper
- added preCreate() lifecycle method that executes before buildRendering().
- fixed a few widgets that were depending on constructor() getting called;
  constructor() doesn't get called anymore with custom elements.
- got rid of widgetId attribute setting on nodes

Fixes #20.