separation and roc plot functions

this is the simplest implementation of the separation plot I could work out. I tried a version with auto-faceting but the call in geom_vline is problematic for that. If you have a way to fix that (everything else works) I'll re-work it and re-issue the pull request. 

I also added a roc plot to the list. 

Let me know if there are any other changes I need to make. Sorry for the messy commit history.