Add slather for test coverage

Computes test coverage using slather. Allows you to [see what code is tested](https://coveralls.io/builds/909181), and also gives you a cute little badge. Just need to enable the repo at https://coveralls.io

[![Coverage Status](https://coveralls.io/repos/marklarr/ObjectiveSugar/badge.png?branch=master)](https://coveralls.io/r/marklarr/ObjectiveSugar?branch=master)
Cool, I'll put something together and update this PR when it's ready :+1: