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.
Won't

```python
w = widgets.FloatSlider(range=True)
```

cause an error when the slider value is changed?