from this instead of this.map. This has practical as well as idealogical
benefits, namely that we can now call other methods on the control from
within a control. This is neccesary for...
2. When the shift key is lifted, stop the zoomBox. This is the way in
which users can *not* zoom once they've started.
git-svn-id: http://svn.openlayers.org/trunk/openlayers@217 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf
