Computed values and some API changes

• Implemented computed values
• Removed dop.observe, dop.unobserve, dop.observeProperty, dop.unobserveProperty
• dop.createObserver is the way to create observers
• Changed the system of observers based absolute paths instead of storing inside of the object
• To observe properties now we have to user observer.observe and pass a string as a name of the property