Cox Regression

### Welcome CoxFitter to the lifelines family! 


The often asked for Proportional Hazards Model has arrived to lifelines! This PR implements both the Efron and Breslows functions for computing the hazard coefficients. Confidence intervals are implemented using the inverse hessian technique. 



### Also, BaseFitter

The superclass `BaseFitter` is now inherited from all fitters: it provides a simple repr function to each one. 

ToDo
 - [ ] Documentation (both RTFD and inline)
 - [ ] More tests
 - [ ] There are lots of `assert`s lieing around - decide what to do with them.
 - [ ] Examples in the documentation too. 
 - [ ] Compare against R, etc.