Rickshaw.Graph.RangeSlider.Preview supports millisecond zooming

This pull requests changes the domainScale interpolate function of Rickshaw.Graph.RangeSlider.Preview to d3.interpolateNumber (vs. d3.interpolateRound) so that the RangeSlider Preview Window can zoom into millisecond levels of precision.

Before it would only allow a window of 1 second which is useless for data such as this (about half a second of data captured at 1kHz)
```
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```