Added Codecov.io coverage reports

Hey Friends! I'm from [Codecov.io](https://codecov.io/) and wanted to personally add our coverage reporting tools to **picasso**! 

You can see the reports **now** here: [codecov.io/github/stevepeak/picasso](https://codecov.io/github/stevepeak/picasso?ref=00fecd35e2723c58ee41935af290aa45f1c4d5a3)

[![codecov.io](https://codecov.io/github/stevepeak/picasso/coverage.svg?branch=master)](https://codecov.io/github/stevepeak/picasso?branch=master)

I'm excited to hear your feedback! Thank you for your time.