revising vignettes for 1.8
found error in predict.mcreg, suspect bug in R itself related
to passing ... and envinronment
cleaned up comments
Tightening up predict.mcreg's declaration
predict method, fixing because of mismatch between R's generic
declaration and mine had newdata as an argument.
typographical corrections in NEWS.
plotPlane has corrections to allow x1lim and x2lim to work properly