Enhanced API and added a spec for ORMultiMap

In addition the removeBindings api call has been removed in favour of using the newly introduced get, put and remove map level operations.
I still think it s wrong to open up for replacing the full `ORSet`. It encourages wrong usage. Replacing an `ORSet` with a new `ORSet` is not the same thing as adding/removing elements to an existing `ORSet`. You loose the history when replacing, and that will result in bad `merge` results for concurrent updates of same entry.

I think you should keep the `ORSet` as a pure implementation detail of the `ORMultiMap`.

Why do you want to add this method in the first place?