Move CharMeasure element to the helper container

Fixes #470

src/xterm.js

@@ -614,7 +614,7 @@ Terminal.prototype.open = function(parent) {
   }
   this.parent.appendChild(this.element);
 
-  this.charMeasure = new CharMeasure(this.rowContainer);
+  this.charMeasure = new CharMeasure(this.helperContainer);
   this.charMeasure.on('charsizechanged', function () {
     self.updateCharSizeCSS();
   });