_ is not defined in timeseries.js (mention Underscore.js dependency in README/docs)

[Winterflower]

Hi timeseries.js team,
I am trying out the basic example in the readme, but hitting the following error in the browser console
"_ is not defined" timeseries.js line 81
This happens when the code hits the _.pluck method, which I believe is part of the Underscore.js library.
It would be nice for the README/docs to mention that Underscore.js is required.