Sample select callback and multi-line X axis labels.

Hi Jonas,
Thanks for making this nice and intuitive way to quickly draw graphs on Android. I am developing an app which is using GraphView and added some features which were useful for me.
My updates are:
1. Provided a callback mechanism to notify user when a sample point is selected.
2. X axis labels can now be multi-line (I needed 2 lines).
This pull request covers both the above changes. Do review and take in if possible.

Thanks,
Narendra