Add Slider component

Implements continuous sliders: https://www.google.com/design/spec/components/sliders.html#sliders-continuous-slider

Ohhh fancy...

![screen shot 2014-11-23 at 7 51 11 pm](https://cloud.githubusercontent.com/assets/637039/5160745/3865b9dc-734a-11e4-84a1-d53db44748f6.png)

Clicking the slider makes the browser select text and `-*-user-select` doesn't seem to work. You guys know of a solution?