Add a zoomslider control.

This pull request adds a zoomslider control to the library.

The control has a default css which can easily be overridden to allow for custom styling as the example shows. The initial position of the thumb matches the resolution of the maps view and updates itself whenever the resolution changes. While dragging the thumb, the resolution of the map gets updated instantly.

Missing functionality (like repositioning of the thumb on container-click) should IMHO be addressed in follow-up pull requests.

Please review.

(Also see #328 for first discussions regarding this pull request)