Implement Semian API

@Sirupsen, @fw42

This implements the API we discussed the other day: https://gist.github.com/Sirupsen/2c3e0fd18b1ee95de877

The hardest part was writing the tests, but I think I've covered all the case we care about. The tests are the part I care most about with regards to reviewing - the implementation was pretty straightforward.

If I understand correctly here, you're receiving a timeout that can be either a `Float` or `Fixnum`, below, you're converting to a double and assuming `ms`. Do we really need `Float` when the unit is `ms`?