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)
I definitely want to make `slather` setup a lot more seamless and less intrusive. The main problem we were running into with pod projects that test their code by including it via a `:path` in the Podfile is that the code goes into the `Pods` project, which people don't normally check in to their project.

So, the quick solution to this was to just hook into the `post_install` cocoapods hook and toggle the Generate Test Coverage and Instrument Program Flow settings for the pod project after it has been generated https://github.com/venmo/slather/blob/master/lib/slather.rb.

Instead of doing things automatically in the Podfile, an alternative solution could be something like

`pod install`
`slather up` (finds the pods project and toggles the settings)
`xcodebuild ... -test`
`slather`