Added Codecov.io coverage reporting

Hey Friends! I'm from [Codecov.io](https://codecov.io/) and I wanted to personally add our coverage reporting tools to retrofit. 

You can see the live reports here: [codecov.io/github/stevepeak/retrofit](https://codecov.io/github/stevepeak/retrofit?ref=1e14167583a64ac3fd18adf2353af60297f4df06) 

[![codecov.io](https://codecov.io/github/stevepeak/retrofit/coverage.svg?branch=master)](https://codecov.io/github/stevepeak/retrofit?branch=master)

I'm excited to hear your feedback! Thank you for your time.