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 :)
Hmm. Difference of interpretation here I guess. The time-column in my data sets, I always interpret as the `death_time`. I am yet to use a dataset where the `birth_times`were not *zero*, but that could happen.

You interpret the time column to be the `durations` though. In case of positive times, and zero birth time:

    duration = death - birth == death

But then I come along and normalize the times. For me, I'm normalizing the death_times (and implicitly the birth_times). Still, the following is true:

    duration = death - birth

But in your interpretation, it doesn't work:

    death = birth + duration

Since now you are getting negative durations.