Allowing custom views for dots

I wanted to have custom dots for the points on the chart, so I added a fairly simple delegate method to do this, which shouldn't interfere much with the existing implementation. Might need some tweaking, let me know what you think. The appearance really depends on the custom view you implement, and you also have to set the background colors for the dots to clear. Not sure if there would be a good way to keep the selection color / normal colour distinctions with this implementation. In the demo app I modified the implementation to show an example.

I haven't forked/pull requested before, so please let me know how I could improve.