Prior Preview Plots

Small R script relying on PEcAn.utils and a new priors#preview verb generate small graphs of priors for table view, and larger versions of those graphs on individual prior view. 

priors/preview/id will return the images/generate them if needed.

Unsure whether to put /public/images/prev in .gitignore or not. On the one hand, takes a while to make all those graphs, but on the other hand they're quite big.