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 PhotoView.

When travis-ci passes you can see the live reports here: [codecov.io/github/chrisbanes/PhotoView](https://codecov.io/github/chrisbanes/PhotoView?ref=f4416ea7c208b56423a9680196b3d62cecb90d5b)

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