Live readout for slider widgets

This implements live slider readout for the `IntSliderWidget` and `FloatSliderWidget`. Here is the look:

![screen shot 2014-01-31 at 9 14 58 pm](https://f.cloud.github.com/assets/27600/2056986/4d977030-8b00-11e3-880c-45cb116814c0.png)