Adding Codecov for Test Coverage to CircleCI yaml (getodk#1462)

* Adds codecov badge to README

* Modifies circle yaml

* Adds jacoco plugin

* Update README.md

README.md

@@ -2,6 +2,7 @@
 ![Platform](https://img.shields.io/badge/platform-Android-blue.svg)
 [![License](https://img.shields.io/badge/license-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
 [![Build status](https://circleci.com/gh/opendatakit/collect.svg?style=shield&circle-token=:circle-token)](https://circleci.com/gh/opendatakit/collect)
+[![codecov.io](https://codecov.io/github/opendatakit/collect/branch/master/graph/badge.svg)](https://codecov.io/github/opendatakit/collect)
 [![Slack status](http://slack.opendatakit.org/badge.svg)](http://slack.opendatakit.org)
 [![codecov.io](https://codecov.io/github/shobhitagarwal1612/collect/branch/master/graph/badge.svg)](https://codecov.io/github/shobhitagarwal1612/collect)