Lifelines

- refactoring of `qth_survival_times`: it can now accept an iterable (or a scalar still) of probabilities in the q argument, and will return a DataFrame with these as columns. If len(q)==1 and a single survival function is given, will return a scalar, not a DataFrame. Also some good speed improvements.
- KaplanMeierFitter and NelsonAalenFitter now have a `_label` property that is passed in during the fit.
- KaplanMeierFitter/NelsonAalenFitter's inital `alpha` value is overwritten if a new `alpha` value is passed
in during the `fit`.
- New method for KaplanMeierFitter: `conditional_time_to`. This returns a DataFrame of the estimate:
med(S(t | T>s)) - s, human readable: the estimated time left of living, given an individual is aged s.
- Adds option `include_likelihood` to CoxPHFitter fit method to save the final log-likelihood value.

- 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)
- pep8 everything

- Ability to specify default printing in statistical tests with the `suppress_print` keyword argument (default False).
- For the multivariate log rank test, the inverse step has been replaced with the generalized inverse. This seems to be what other packages use.
- Adding more robust cross validation scheme based on issue 67.
- fixing `regression_dataset` in `datasets`.

- `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.
- more sanitizing for the statistical tests =)

- `CoxFitter` implements Cox Proportional Hazards model in lifelines.
- lifelines moves the wheels distributions.
- tests in the `statistics` module now prints the summary (and still return the regular values)
- new `BaseFitter` class is inherited from all fitters.