Added Codecov.io coverage reports

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