Improve support for subviews

* subviews can now be lazily created by
  leap view using factories provided in subviewCreators
* createSubview replaces and renders subviews
* renderSubview is an evolution of assign where
  passing a list of subview names is enough to render them
Merge pull request #2 from KidkArolis/improved-subviews

Improving the subview support via subviewCreators, createSubview and ren...