add a way to destroy tokens

fixes #83 

@warner or @fmarier r?
You might consider constructing this "wrong secret" by flipping the low-order bit of the right secret. It's not a serious concern, obviously, (if it were, we'd have bigger problems to deal with), but at least technically there's a non-zero chance that this random secret would match the right one, so sometimes I find it more clear to make it explicitly different. (it'd make an actual difference only if the value were much much shorter, like if we were assigning 16-bit UIDs or something, in which case you'd get occasional random test failures that would be kinda annoying)