ENH: Implement a range slider widget.

This uses a jQuery range slider to specify a range (two numbers) using one slider, an `IntRangeSliderWidget`. This is a simple and useful extension of `IntSliderWidget`. For example, I will use it to set thresholds for filtering data (e.g., the upper and lower cutoffs of a band pass).

Is this of general enough use to introduce to IPython? If so, a `FloatRangeSilderWidget` should be implemented as well.