Adding conditional time to

####0.4.3
 - refactoring of `qth_survival_times`: it can now accept an iterable (or a scalar still) of probabilities in the q argument, and will return a DataFrame with these as columns. If len(q)==1 and a single survival function is given, will return a scalar, not a DataFrame. Also some good speed improvements.
 - KaplanMeierFitter and NelsonAalenFitter now have a `_label` property that is passed in during the fit.
 - KaplanMeierFitter/NelsonAalenFitter's inital `alpha` value is overwritten if a new `alpha` value is passed
 in during the `fit`.
 - New method for KaplanMeierFitter: `conditional_time_to`. This returns a DataFrame of the estimate:
    med(S(t | T>s)) - s, human readable: the estimated time left of living, given an individual is aged s.
- Adds o