0.8.0

• reorganized lifelines directories:
  • moved test files out of main directory.
  • moved utils.py into it's down 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.