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 scribe-java.

You can see the live reports here: [codecov.io/github/stevepeak/scribe-java](https://codecov.io/github/stevepeak/scribe-java?ref=)

[![codecov.io](https://codecov.io/github/stevepeak/scribe-java/coverage.svg?branch=master)](https://codecov.io/github/stevepeak/scribe-java?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.