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.
> hmm, entries will still cast to ORSet

It did before also. An `ORMultiMap` should only have elements of `ORSet`, otherwise it is not a multi map. However I will ensure that `put` may only accept `ORSet` as its value.