move plyr package to Imports

to avoid plyr interfering with dplyr in the user session
use integrate() for calcProbXGreaterThanY()

use R's built-in integrate() to evaluate P(X>=Y)
fix calcProbXDiffNormals() for very small P-values

Instead of returning 1-P(X<=Y), when P(X<=Y) is close or equals to 1,
calculate P(Y<=X) directly. That fixes potential cases, when the
resulting P(Y<=X) would be 0.0 due to the loss of precision.
Added calcProbXGreaterThanY.orig for control over integration
Updated vignette