changed selection of features from p-values to q-values

rather than using a p-value threshold for selection of significant
features (which is prone to multiple testing problems), all
p-values are converted to q-values and selection is based on the
FDR.

The CLI parser has been modified to allow the use in most cases to
select the FDR cutoff. Otherwise, a value of 0.05  is used..

The graphing of p-value distribution and pi0_hat vs lambda has
been improved for clarity.