Removing can be inefficient (polynomial time)

This is a very useful library.
Maybe you should note somewhere, that removing can be inefficient (if a value is removed which is a value for every key). At first you iterate the value-key map and then you perform indexOf and splice (so you could have cubic time in the worst case).

But I guess this is due to the nature of the problem (or maybe some smart computer scientists find an efficient way).
