v0.4.1

 - `CoxFitter` is now known as `CoxPHFitter`
 - refactoring some tests that used redundant data from `lifelines.datasets`. 
 - Adding cross validation: in `utils` is a new `k_fold_cross_validation` for model selection in regression problems.
 - Change CoxPHFitter's fit method's `display_output` to `False`.
 - fixing bug in CoxPHFitter's `_compute_baseline_hazard` that errored when sending Series objects to
   `survival_table_from_events`.
 - CoxPHFitter's `fit` now looks to columns with too low variance, and halts NR algorithm if a NaN is found.
 - Adding a Changelog.