Update default control styles and provide option to disable them

This PR updates the default control styles to match those in the recent Google Maps release.

Additionally, this adds a way to disable default control styles by passing an additional option:

```javascript
map.addControl({
  position: 'top_right',
  content: '<i class="icon-location"></i>',
  disableDefaultStyles: true
});
```

Why? Because in order to do *truly* custom controls, it's likely that you'll override the defaults (`font-family`, `font-size`, `box-shadow`) anyways. As it stands it's necessary to use `!important` in your stylesheet in order to override those inline styles… ugly. A common use case is demonstrated above: using an icon in place of text content for a marker.