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 

[![Coverage Status](https://coveralls.io/repos/marklarr/MagicalRecord/badge.png?branch=develop)](https://coveralls.io/r/marklarr/MagicalRecord?branch=develop)