Added checkstyle rule that warns about tabs in the code

Author: Knut Anders Hatlen

checkstyle/style.xml

@@ -8,6 +8,7 @@
         <property name="headerFile" value="checkstyle/fileheader.txt"/>
     </module>
     <module name="FileLength"/>
+    <module name="FileTabCharacter"/>
     <module name="TreeWalker">
         <module name="AvoidStarImport"/>
         <module name="RedundantImport"/>