Various bug fixes; implement 'is' binding (upgrades an element to a component)

- Implement ability to prevent property events from bubbling (skin property should not bubble)
- 'foreach' binding should insert and remove the correct elements, even if the template consists of more than one child node
- ComputedObservable should not regenerate its value after its dispose method has been invoked
- Orientation should use textual values not numbers so that it can be used from templates