Allow "_" in package names

Author: dmikurube

Diff for: config/checkstyle/checkstyle.xml

@@ -171,7 +171,7 @@
       <property name="option" value="nl"/>
     </module>
     <module name="PackageName">
-      <property name="format" value="^[a-z]+(\.[a-z][a-z0-9]*)*$"/>
+      <property name="format" value="^[\_a-z]+(\.[\_a-z][\_a-z0-9]*)*$"/>
       <message key="name.invalidPattern"
              value="Package name ''{0}'' must match pattern ''{1}''."/>
     </module>