Four two

### 0.4.2

 - Massive speed improvements to CoxPHFitter. 
 - Additional prediction method: `predict_percentile` is available on CoxPHFitter and AalenAdditiveFitter. Given a percentile, p, this function returns the value t such that *S(t | x) = p*. It is a generalization of `predict_median`. 
 - Additional kwargs in `k_fold_cross_validation` that will accept different prediction methods (default is `predict_median`).
 - Bug fix in CoxPHFitter `predict_expectation` function. 
 - Correct spelling mistake in newton-rhapson algorithm.
 - `datasets` now contains functions for generating the respective datasets, ex: `generate_waltons_dataset`.
 - Bumping up the number of samples in statistical tests to prevent them from failing so often (this a stop-gap)

@spacecowboy, any comments? 
Is there actually a predict_mean function ?