Add rule enforcing use of spaces, not tabs.

Author: Quintin Willison

config/checkstyle/checkstyle.xml

@@ -6,6 +6,8 @@
 <module name="Checker">
 	<property name="charset" value="UTF-8"/>
 
+	<module name="FileTabCharacter"/>
+
 	<module name="TreeWalker">
 		<module name="RedundantModifier"/>
 	</module>