Use FindBugs Maven plugin and fix FindBugs errors.

Run findbugs:check at the before_script phase of the Travis build.
The current version of findbugs-maven-plugin does not work with Java
1.8, so the check is not included in the maven compile phase for now.

This patch also fixes some javadoc format errors.
Merge pull request #19 from m1ch1/findbugs

Use FindBugs