Automatic recreating after changing the window size

Dear ui-chart team,

on fixed windows sizes (tablets, ...) it doesn't make a lot of sense, but for a normal desktop environment an automatic recreation of the plot canvas would be very useful if the user resizes the window.

Adding

```
    $(window).resize(function(){
      renderChart();
    });
```

directly after the linking function of the directive would solve the issue in a very simple way.
