added moveBackToLineStart() method on ZString.

ZString.cpp

@@ -583,6 +583,14 @@ namespace coreutils {
 	  ++line;
       return line;      	   
    }
-   
-   
+
+   void ZString::moveBackToLineStart() {
+      while(cursor != data) {
+	 if(*cursor == '\n')
+	   break;
+	 cursor--;
+      }  
+      return;
+   }
+      
 }