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 zxing. 

        You can see the live reports here: [codecov.io/github/stevepeak/zxing](https://codecov.io/github/stevepeak/zxing?ref=e061224d80a9de1ff81f438b2d66645d95c8de75) 

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

        I'm excited to hear your feedback! Thank you for your time.