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 metrics.

When travis-ci passes you can see the live reports here: [codecov.io/github/dropwizard/metrics](https://codecov.io/github/dropwizard/metrics?ref=fe94ba4b1166ceb2d36e98d6e026223e09745b44)

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