documentation small fixes
Enforce D coded as 0 and 1

In the calculate_roc function, D is assumed to be coded as 0 and 1, added a check in the code and updated the argument description.
Merge pull request #15 from ecpolley/master

Enforce D coded as 0 and 1
title case for CRAN
Merge branch 'master' of https://github.com/sachsmc/plotROC
Allow arbitrary class labels, fixed vignette, added some tests, and edited documentation
Merge pull request #19 from sachsmc/master

Lots of small updates