Major adaptation of the ggs_rocplot() function more according to plyr tools, and also including the possibility of being fully bayesian.
Force sorting the roc dataframe to be sure that the line is sorted.
Some improvements:

  - Object is sorted to be sure that the curve is fine.

  - Use geom_step() instead of geom_line()

  - Add geom_rug()

  - Code readability.
Rewrite ggs_rocplot() to take out the roc.calc() function. Clean also the documentation.
Merge branch 'me-roc'

Conflicts:
	R/ggs_roc.R

Rename the file, rerun the documentation and clarify the merge.