Fixed all Findbugs warnings

Also added findbugs:check to the Maven build to ensure no violations are added now that everything is clean.

Added guava & findbugs-annotations to the Ant build to preserve compatibility. It seems that deb / rpm generation is only available through Ant at the moment, so let's keep this working until those packages are also generated by Maven.
Merge pull request #170 from jmxtrans/fix-findbugs-errors

Fixed all Findbugs warnings