Fix issue with multiple toolbars appearing on each image, ensuring that each image only ever has a single attached toolbar.

Addresses manleyjster/Leaflet.Toolbar/issues/2.  After taking a look a the Leaflet.Toolbar code, I decided that this was an issue for application code, rather than library code.

It was a small fix, really.  In `DistortableImage.Edit#_showToolbar`, I added:

```javascript
if (this.toolbar) {
    map.removeLayer(this.toolbar);
}
```