Added Codecov integration

@cainus it would be my honor for you to proof this pr's validity. Thank you!

```sh
curl -X HEAD https://codecov.io/github/stevepeak/timestring/coverage.png -D -

HTTP/1.1 302 Found
X-Coverage: 54
Cache-Control: no-cache
Location: https://s3-us-west-2.amazonaws.com/codecov.io/media/images/coverage/png/54.png
```