Camelcase all keys

Working with Mapbox style spec objects in Javascript is less ergonomic because all the dashcase properties must be quoted when they are used as identifiers. 

```js
var layer = { 'line-color': 'blue' }
layer['line-color']

vs

var layer = { lineColor: 'blue' }
layer.lineColor
```

They look odd in conjunction with the camelcased source properties #175 