Lifelines

- deprecates Pandas versions before 0.18.
- throw an error if no admissable pairs in the c-index calculation. Previously a NaN was returned.

- add two summary functions to Weibull and Exponential fitter, solves 224

- new prediction function in `CoxPHFitter`, `predict_log_hazard_relative_to_mean`, that mimics what R's `predict.coxph` does.
- removing the `predict` method in CoxPHFitter and AalenAdditiveFitter. This is because the choice of `predict_median` as a default was causing too much confusion, and no other natual choice as a default was available. All other `predict_` methods remain.
- Default predict method in `k_fold_cross_validation` is now `predict_expectation`

- supports matplotlib 1.5.
- introduction of a param `nn_cumulative_hazards` in AalenAdditiveModel's `__init__` (default True). This parameter will truncate all non-negative cumulative hazards in prediction methods to 0.
- bug fixes including:
- fixed issue where the while loop in `_newton_rhaphson` would break too early causing a variable not to be set properly.
- scaling of smooth hazards in NelsonAalenFitter was off by a factor of 0.5.

- reorganized lifelines directories:
- moved test files out of main directory.
- moved `utils.py` into it's own directory.
- moved all estimators `fitters` directory.
- added a `at_risk` column to the output of `group_survival_table_from_events` and `survival_table_from_events`
- added sample size and power calculations for statistical tests. See `lifeline.statistics. sample_size_necessary_under_cph` and `lifelines.statistics. power_under_cph`.
- fixed a bug when using KaplanMeierFitter for left-censored data.

- addition of a l2 `penalizer` to `CoxPHFitter`.
- dropped Fortran implementation of efficient Python version. Lifelines is pure python once again!
- addition of `strata` keyword argument to `CoxPHFitter` to allow for stratification of a single or set of
categorical variables in your dataset.
- `datetimes_to_durations` now accepts a list as `na_values`, so multiple values can be checked.
- fixed a bug in `datetimes_to_durations` where `fill_date` was not properly being applied.
- Changed warning in `datetimes_to_durations` to be correct.
- refactor each fitter into it's own submodule. For now, the tests are still in the same file. This will also *not* break the API.