tolerate configurable amount of clock skew

This is not yet complete, but should be a good direction to go for adding tolerance of clock skew. It still needs a way to get the 'options' value into the `verify()` call, and some tests.

@seanmonstar, is the way I'm testing for function-ness of `cb` the right way to do it? I'm still a bit fuzzy on this crazy argument polymorphism that the JS world seems to think is normal :).

refs #58 and #59