[BUG]: RTL column menu wrong positioning

Here's the plnkr: http://plnkr.co/edit/MAjaBO3Bpe3mf04JxWpo?p=preview
Simply open one of the column menus and see it doesn't show in the right place.

Using this code in the `repositionMenu` function:

```
if (column.grid.isRTL()){
        left = positionData.left + positionData.parentLeft+ positionData.width;
}
```

Seems to fix this.
