allow for specification of time axis as utc

Submitting for comments. Likely a long way from being merged. 

I need the tick marks on a time axis to render in the UTC timezone. The current behavior renders labels and ticks on the time axis in the browser's timezone. d3 has good support for this, but it is not exposed in the Plottable API. This PR seeks to augment the Plottable API to allow specification of UTC for the time axis.