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.
Yes, `ORMap` is more general than `ORMultiMap`, but then so is `Map` when compared with `MultiMap` in Scala's collections. Perhaps we should implement `ORMultiMap` in exactly the same way that `MultiMap` is mixed in.

Perhaps we should just hangout in an hour or so and discuss. WDYT?