added moveBackToLineStart() method on ZString.

ZString.h

@@ -445,7 +445,14 @@ namespace coreutils {
       ///
 
       int getLineNumberAtCursor();
-      
+
+      ///
+      /// Move the cursor to the beginning of a line or to the beginning of
+      /// the ZString.
+      ///
+
+      void moveBackToLineStart();
+            
     protected:
       char *data;
       char *cursor;