allows you to specify a level. This will need to be refactored: Possibly the zoom control will become part of a
subclass of PanZoom, "largePanZoom", which has a smaller set of controls.
Also add a DEFAULT_ZOOM_LEVELS setting, and an accesssor for said setting.
git-svn-id: http://svn.openlayers.org/trunk/openlayers@23 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf