Added Namespace#getEnum and changed #lookupEnum to the same behavior, see #576

Author: dcodeIO

LICENSE

@@ -1,6 +1,6 @@
-This license applies to all parts of protobuf.js except those files explicitly
-including or referencing a different license or files located in a directory
-containing a different LICENSE file.
+This license applies to all parts of protobuf.js except those files either
+explicitly including or referencing a different license or located in a
+directory containing a different LICENSE file.
 
                                  Apache License
                            Version 2.0, January 2004