Make project compile under Java 8.

Most of the errors were coming from stricter rules for javadoc. Findbugs was upgraded
to version 3.0.0, and PowerMock to 1.5.4 to have Java 8 support.
Additionally gradle wrapper was updated to the latest 1.x version.
Minor updates.
Downgrade to Findbugs 2.0.1 if build run with Java 6.
Merge pull request #135 from tbak/master

Make project compile under Java 8.