Integrate code coverage to build and review process

We cannot use codecov, but we can use coveralls. We have the maven plugin included in the pom and need to invoke it appropriately in our various builds, and disseminate knowledge about browser extensions to get it into the pull request UI.

Imported from Jira [BEAM-177](https://issues.apache.org/jira/browse/BEAM-177). Original Jira may contain additional context.
Reported by: kenn.

Start with the repository's pom and identify the existing Maven coverage plugin configuration, then trace how the various builds are invoked. Determine how coverage should reach the pull request UI and what browser-extension guidance must be documented. Done means the builds invoke Coveralls appropriately and the integration knowledge is available to reviewers.

Written by the indexing model from the issue text.