YANG Derived Data Type Support + Change in YANG Parser Grammar to 
Support String values which are not in double quotes with UT case + 
Addition of Revision Date Validation with UT case + Removal of 
String.valueOf

Change-Id: Ic152265a35c0d2b0f84473a94ca67681cf8efca8
diff --git a/src/main/resources/GeneratedYang.g4 b/src/main/resources/GeneratedYang.g4
index 428bd91..b3272bc 100644
--- a/src/main/resources/GeneratedYang.g4
+++ b/src/main/resources/GeneratedYang.g4
@@ -1220,4 +1220,6 @@
                            defaultStatement? configStatement? mandatoryStatement? minElementsStatement?
                            maxElementsStatement? RIGHT_CURLY_BRACE));
 
-    string : STRING (PLUS STRING)*;
+    string : STRING (PLUS STRING)*
+           | IDENTIFIER
+           | INTEGER;