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.

Do you think FNV would potentially be better? Probably not the case, but at least it's designed to generate a 32 bit output.

It would be nice to have some form of collision detection, but I'm not sure how that would work.