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 :)
Why the `assert`? I'm half punting the problem, and half stating that there is no good solution. ¯\_(ツ)_/¯