Range widgets

The jquery slider supports range (2-handle) sliders, which meet a fairly regular use-case when doing interactive graph plotting (adjusting axis limits, filtering windows, etc).

This adds `Int` and `FloatRangeSlider` widgets (by minimal modification of the existing slider widgets), and extends the `interact()` widget builder to interpret 4- or 5-number tuples as `(min, low, high, max, [step])` to construct the appropriate range widget. Test cases also included.
True, the 4/5-tuple is probably a bit opaque, but having to write it out fully: `IntRangeSliderWidget(...)` seems a bit unwieldy also. 

Just `Range(...)` would probably be too ambigious with `range(...)` to have in the namespace, but I was wondering if interpreting tuples like `param=("range", 100, 200)` might be better to make it explicit what is being constructed, but it doesn't support named parameters so the problem of remembering the ordering of arguments remains. Any other options?