Added ability to place navigator with scrollbar at top of chart

Now it is possible to place navigator with or without scrollbar, or a scrollbar only at top of the chart using this options: 
```javascript
scrollbar: {
    opposite: true, //place scrollbar with navigator at top
    oppositeMargin: 5 //with this option it is possible to move scrollbar with navigator highter or lower to have good look.
}
```
Also aditional lines drawn at bottom of navigator when it is on top: it becomes more beautiful.