Destroy chart when scope gets destroyed

Hi,

I'm currently creating an Angular app using highcharts-ng to draw different types of graphs. Today I noticed that my app was leaking quite much memory when I switched between views/routes. I had a look at highcharts-ng code, and saw that the chart does not get destroyed if scope is destroyed.

I created a simple test case app to see if I can verify the increased memory usage. I added a few routes to load 1-3 simple bar graphs to the test page each time a link got clicked.

Without this fix the memory usage would always stabilize itself to ~9.5mb after GC runs, but with this fix the memory usage was always ~8.4mb after memory was cleaned. So, over 1mb of memory savings for a very simple test app.