Bumped the version of findbugs to 3.0.0.

1. Main motivation is that 3.0.0 works with java8.
2. Reworked the integ test coverage so that it works with java 6,7,8. Next step is to add reasonable error message if someone attempts to run findbugs plugin with old version of java (e.g java6 and lower).
Fixed the problem with findbugs integration test. Ensured it works with java6 cleanly.
Added validation for FindBugs classpath depending on the currently used java version.
Tidy-up
Added an integration test for FindBugs classpath validation. Fixed a problem with accessing a property (ha, one of the convention mapping gotchas ;).
Tidy up in the integration tests for FindBugs. Tweaked the exception messages.
Mentioned in the release notes that we have bumped FindBugs version to 3.0.0