performance improvements for CoxPHFitter
- `WeibullFitter` has renamed the incorrect `_jacobian` to `_hessian_`.
- `variance_matrix_` is now a property on fitted `WeibullFitter` which describes the variance matrix of the parameters.
- The default `WeibullFitter().timeline` has changed from integers between the min and max duration to _n_ floats between the max and min durations, where _n_ is the number of observations.
+ - Performance improvements for `CoxPHFitter` (~15% faster)
#### 0.14.6
- fix for n > 2 groups in `multivariate_logrank_test` (again).
