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.

Yeah, totally is - I'll fix that (from `semget(2)`):
> (though the execute permissions are not meaningful for semaphores, and write permissions mean permission to alter semaphore values)