Clarify presentation of test functions

Clarify relationship between the observed value (to which null hypothesis
relates), the test statistic, and the confidence interval.

- 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 summary
- unify t-test code
Separate Wilcoxon and Mann-Whitney test

These two tests use similar functionality but are actually different.

- add common file to extract helper functions
- clean up Wilcoxon and Mann-Whitney
- adapt function to pretty print
Add meta infos to circular tests
Merge pull request #13 from BigCrunsh/add_test_statistic

Clarify presentation of test functions and parameters