Check for unused imports with CheckStyle

* This also provides a fast way to check for unused imports with:
  $ jt checkstyle

Author: eregon

src/main/.checkstyle_checks.xml

@@ -34,6 +34,7 @@
       <property name="format" value="^[A-Z][_a-zA-Z0-9]*$"/>
     </module>
     <module name="RedundantImport"/>
+    <module name="UnusedImports"/>
     <!--<module name="LineLength">-->
       <!--<property name="max" value="250"/>-->
     <!--</module>-->