Add Checkstyle check for trailing whitespace

Author: garydgregory

src/conf/checkstyle.xml

@@ -148,4 +148,11 @@
     <property name="severity" value="warning" />
   </module>
 
+  <module name="RegexpSingleline">
+    <property name="format" value="\s+$" />
+    <property name="minimum" value="0" />
+    <property name="maximum" value="0" />
+    <property name="message" value="Line has trailing spaces." />
+  </module>
+
 </module>