Fix incorrect number of steps in Torque

When using torque with small datasets, it is noticeable that the animation has one more step than it should, glitching just before it ends. This is caused by not adding 1 on the first/last date differential, when calculating the step ratio (date differential/number of steps).

Here's the glitch:
![mal](https://cloud.githubusercontent.com/assets/3707222/5395635/5e46c948-814b-11e4-990c-e6b9f0c3736f.gif)

And here's the same vis, with the appropriate diff.
![bien](https://cloud.githubusercontent.com/assets/3707222/5395628/51395a40-814b-11e4-8964-55888606a408.gif)

(cc @javisantana)