Added a Wilcoxon signed-rank test based stopping condition.

It statistically compares the performance of two NNs.
The code is adapted from the implementation in R-2.10.1
Some pre-merge editing.

Fixed the header in NNPerformanceChangeStoppingCondition.

Made static functions public and removed unneeded annotations in
OneTailedWilcoxonNNPerformanceComparator.