Added Codecov.io coverage reporting

Hey Friends! I'm from [Codecov.io](https://codecov.io/) and I wanted to personally add our free coverage reporting tools to HoloEverywhere.

When travis-ci passes you can see the live reports here: [codecov.io/github/Prototik/HoloEverywhere](https://codecov.io/github/Prototik/HoloEverywhere?ref=e463f84e254b3d9f949e5e6362a5329d5c53ea7d)

[![codecov.io](https://codecov.io/github/Prototik/HoloEverywhere/coverage.svg?branch=master)](https://codecov.io/github/Prototik/HoloEverywhere?branch=master)

Make sure to [signup](https://codecov.io/login/github) to enable these free [features](https://codecov.io/#features)

I'm excited to hear your feedback!
Thank you for your time.