Rickshaw.Graph.RangeSlider.Preview supports millisecond zooming

Changed the domainScale interpolate function to d3.interpolateNumber
(vs. d3.interpolateRound) so that the RangeSlider Preview Window could
zoom into millisecond levels of precision (before it would only allow a
window of 1 second)
Merge pull request #372 from deldrid1/master

Rickshaw.Graph.RangeSlider.Preview supports millisecond zooming