Add Kruskal-Wallis test
Emphasise approximation and clean-up

- change copyright
- use tiedrank_adj from wilcoxon.jl to be more consistent and efficient
- replace split helper function by explicitly looping to be more
  efficient
- add documentation to bring asymptotic approximation to attention and
  add warning for small group sizes.
Merge pull request #14 from BigCrunsh/add_kruskal_wallis

Add Kruskal-Wallis test