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 butterknife.

When travis-ci passes you can see the live reports here: [codecov.io/github/JakeWharton/butterknife](https://codecov.io/github/JakeWharton/butterknife?ref=10ec4d641ba308507154859785cdaa5a0ae27010)

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