Add help and tutorial

[cdeil]

For now, we should add a help button next to the about button that opens up an overlay popup in the middle similar to http://www.chromoscope.net/ with some info on how to use the web page.

Later we could add a video showing how to use it (explore a few objects or regions) or a tour how to use the different widgets using http://bootstraptour.com/ or one of the other available options: http://ninodezign.com/25-free-jquery-plugins-for-doing-guided-tours-through-a-website/