Add a getter on the Highcharts chart object

I think this is very useful because it enables us to use the Highcharts api without having to use jQuery to get the reference.
However, I'm not really sure what the correct implementation should be. Any comments is welcome.

In my project, I've made the change present in this pull request and it is working well: if the ```$scope.myChart``` is passed to the directive using the config attributes, I can simply do ```$scope.myChart.getHighcharts()``` to get the reference on the real Highcharts object...