Make unbind more consistent with `key`

Previously, the `key` function and `unbind` function were assymetrical
so the following ignored the clear meaning intended:

    key('a, b', function() { alert("You Hit A or B!") });
    # User Interaction...
    key.unbind('a, b')

Only `a` was unbound.

The code was almost 100% there; I just mirrored the behavior I saw in
the `assignKeys` function.

The code is tested (failing tests were produced first).