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
WIP Approach to clarify presentation of test functions

Currently, some of the basic concepts are not presented in the best
way; it is a bit cumbersome to distinguish between the observed
value to which the null hypothesis relates, the test statistic,
and the confidence interval. The goal of this commit is to make clear
how these quantities belong together and to ensure a consistent
implementation over all tests. Therefore, some suggestions:

- added and printed out meta function "population_param_of_interest"
  to clarify between observation w/ confidence interval and actually
  test statistic.
- add show_params to describe meta data of a test (e.g., contingence
  table) and unify pretty print
- group point estimate and confidence interval to test statistic
- unify t-test code
- adapt test to pretty print