Move background-image from anchor to new span element to allow bigger buttons

Hi, currently leaflet-draw buttons cannot be made bigger because the background images will break:
- Add span element with class 'icon' inside the anchor
- Change CSS: move background from anchor to span element
- Now the anchor (button) can be bigger and the icon can stay the same

Example:
.leaflet-draw-toolbar a, .leaflet-draw-toolbar a:hover { width: 52px; height: 52px; }
.leaflet-draw-toolbar a .icon { margin: 13px }	

Could you please remove the unnecessary code, it should probably be:

````js
L.DomUtil.create('span', 'icon', link)
````