Add some code documentation

[ameshkov]

I'd appreciate if you add a comment in the file head describing the logic starting from the entry point (DeepOverrideHost.addProperty) and briefly explaining the algorithm.

Something like this:

1. The entry point -- addProperty, accepts this and that parameter
2. Then we're building the object state tree for the given path (buildAbstractStateTree)
3. Using this tree we're applying the object state and override necessary properties (applyObjectState)
4. Etc, etc

This will greatly help the reader to understand the code.