Add code quality plugins to build.gradle (jacoco, findbugs)

[davidmoten]

This is one thing I've missed for a while so I figured out how to get gradle to provide them. This PR enables production of Jacoco coverage reports and Findbugs reports as html in the build directory. It doesn't happen by default, the plugins are run like this (instructions in comments in build.gradle):

To run Findbugs:
./gradlew check

To run Jacoco test coverage:
./gradlew test jacocoTestReport

To run Jacoco on a single test:
./gradlew -Dtest.single=OperatorRetry test jacocoTestReport

Then open up the reports in build/reports.

[benjchristensen]

Looks like reasonable additions, and safe since they don't affect normal builds.

Thanks.