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.

Same question about using a `cond`.