Optional event_col in AAF and CPH fitters

@spacecowboy, this PR adds the option for the `event_col` parameter (default None). If left as `None`, then we assume all non-censored (similar to the kmf fitter). 

Also, `duration_col` is now a mandatory argument. 
One thing that Pandas doesn't have that I would really like to see in future releases is something similar to `ones_like` in Numpy. Ex:

```
s = pd.Series( [True, True, False], index=['A', 'B', 'C'], dtype=float)
print pd.ones_like(s)
"""
A    1
B    1
C    1
dtype: float64
""""