Fix serious bug in Wilcoxon T test

In this test we should only count distinct pair. Equal pairs were
discarded during calculation of T+,T- but sample size weren't adjusted
Add expression for the normal approximation for large sample sizes
Put paper reference to separate secrtion
Use normal approximarion for calculation of critical value and significance

Fixes #18
Use new API for Wilcoxon T test
Reorder import list
Port Kruskal-Wallis test to new API
remove kruskalWallisSignificant