Added more ggplot2 examples in extras/ #mozsprint

Added more material and examples to the ggplot2 lesson in extras.

* dealing with overplotting 
  * alpha
  * geom_hex
  * stat_density2d
* adding new layers to plots
* geom_jitter
* geom_boxplot
* geom_violin
* geom_smooth
* plotting statistics derived from data with stat_summary
* combining aesthetics, mappings, scales, other options to create non-traditional graphics, with notes of caution about creating senseless visualizations.