Fixing negative times

This address #105. There were two separate issues: one easy and one a little deeper. 

1. In plotting, I assumed the min time was 0, this has been fixed.
2. I changed some logic in the core `survival_times_to_table` function that makes it more robust. 

I've added tests for both fixes. @spacecowboy for review please :)
In your data, after normalization, your durations are sometimes negative. So the first condition would fail too often. 

>  It seems to me that the following is always true:

Do you mean always *positive*? I can't guarantee that `death_times` is always positive, but perhaps I should restrict that as a more clean (and loose) assertion.