Widget infrastructure and zoom slider

See [here](http://opengeoscience.github.io/geojs/examples/ui.html) to test it out.  The style is the best my meager graphic design skills could come up with.  I would be happy to hear ideas about how to improve it.

For now, the slider is not enabled by default.  To add the slider:
```javascript
map.createLayer('ui').createWidget('slider');
```

My plan is to add all widgets to a single uiLayer.  For now the base classes are minimal until I figure out what functions are generically useful.