Added handling for YANG extensions related to RFC 6536 (default-deny-write, default-deny-all) and RFC 6241 (get-filter-element-attributes) and added in Unit Tests for the new Listener classes related to extension references

Change-Id: I6501c23d340f0d6f85ecbe8790186568152d341f
diff --git a/compiler/base/parser/src/main/resources/YangLexer.g4 b/compiler/base/parser/src/main/resources/YangLexer.g4
index 45189f7..cd942a0 100644
--- a/compiler/base/parser/src/main/resources/YangLexer.g4
+++ b/compiler/base/parser/src/main/resources/YangLexer.g4
@@ -113,6 +113,11 @@
     DATA_STRUCTURE_KEY : IDENTIFIER COLON KEY_KEYWORD;
     APP_EXTENDED_KEYWORD : 'app-extended-name';
     APP_EXTENDED : IDENTIFIER COLON APP_EXTENDED_KEYWORD;
+    //From ietf-netconf-acm.yang RFC 6536
+    DEFAULT_DENY_WRITE_KEYWORD : 'default-deny-write';
+    DEFAULT_DENY_WRITE : IDENTIFIER COLON DEFAULT_DENY_WRITE_KEYWORD;
+    DEFAULT_DENY_ALL_KEYWORD : 'default-deny-all';
+    DEFAULT_DENY_ALL : IDENTIFIER COLON DEFAULT_DENY_ALL_KEYWORD;
 
     // Lexer tokens to be skipped
     COMMENT
@@ -147,4 +152,4 @@
     fragment ALPHA      : [A-Za-z];
     fragment DIGIT      : [0-9];
     fragment URN        : [u][r][n];
-    fragment HTTP       : [h][t][t][p];
\ No newline at end of file
+    fragment HTTP       : [h][t][t][p];