Lifelines 0.5.1

• New API for CoxPHFitter and AalenAdditiveFitter: the default arguments for event_col and duration_col. duration_col is now mandatory, and event_col now accepts a column, or by default, None, which assumes all events are observed (non-censored).
• Fix statistical tests
• Allow negative durations in Fitters
• New API in survival_table_from_events: min_observations is replaced by birth_times (default None).
• New API in CoxPHFitter for summary: summary will return a dataframe with statistics, print_summary() will print the dataframe (plus some other statistics) in a pretty manner.
• Adding "At Risk" counts option to univariate fitter plot methods, .plot(at_risk_counts=True).
• Fix Epanechnikov kernel.