Add p-values to the ProbABEL output

[lckarssen]

This is a 'liftover' of feature request #2086 from the R-forge tracker.

At the moment the ProbABEL output contains the values for beta and se_beta and sometimes the log-likelihood.
Various people have requested to add p-values as well. See for example this discussion on the GenABEL forum: http://forum.genabel.org/viewtopic.php?f=10&t=605&p=1058&hilit=value#p1058

[lckarssen]

See the work in the ProbABEL-pvals branch.