add reverse methods

[bblodfon]

@RaphaelS1 good to merge!

• I added the reverse also in the autoplot.Task function, seems useful.
• crank default predict type for CoxPH (#299)

Btw, when you run document() do you get the surv.kaplan and surv.rpart docs re-written as follows? ie:

+\item lp is predicted 1.37095844714667 * lp is predicted -0.564698171396089 * lp is predicted 0.363128411337339 * lp is predicted 0.63286260496104 * lp is predicted 0.404268323140999