check Spotbugs code with FindBugs
check Spotbugs code with FindBugs

- resolves #82
- using Spotbugs to check itself would be better but needs appropriate Gradle plugin,
see https://github.com/gradle/gradle/pull/2538 and #15

sewe/spotbugs,spotbugs/spotbugs,sewe/spotbugs,spotbugs/spotbugs,KengoTODA/spotbugs,KengoTODA/spotbugs,sewe/spotbugs,spotbugs/spotbugs,sewe/spotbugs,KengoTODA/spotbugs,KengoTODA/spotbugs,spotbugs/spotbugs,spotbugs/spotbugs