Summaries + datetime_to_duration bug fix

[CamDavidsonPilon]

This adds two summary functions to Weibull and Exponential fitter, solves #224

from lifelines import WeibullFitter
w = WeibullFitter()
s = np.random.weibull(2.5, size=10000)
w.fit(s)
w.summary

             coef  se(coef)  lower 0.95  upper 0.95
lambda_  1.004762  0.004245    0.996441    1.013083
rho_     2.490955  0.019517    2.452694    2.529215

[CamDavidsonPilon]

Some test errors unrelated to this PR due to pandas 0.18. I'll fix that first and bump version.