[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. This is a first attempt to demonstrate the conceptual idea. Specifically, 

- added and printed out meta function "statistic" to clarify between observation w/ confidence interval and actually test statistic.
- adapt pretty print
- 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
- some reordering of functions
- unified t-tests

@simonster: What do you think?
Agreed that parametrizing `df` doesn't make sense (9223372036854775808 samples ought to be enough for anybody), but it would be useful to parametrize `xbar`, `stderr`, `t`, and `μ0` (can probably all be the same type parameter `T<:Real`).