new methods to disable/enable the geosearch control programmatically

[jgravois]

the idea for this PR came from a request for assistance on gis.stackexchange

the new L.esri.Geocoding.geosearch.disable() method removes the click event handler and grays out the control.

changes:

1. use a named private function when hooking up the event listener to determine what happens when a user clicks on the wrapper div (so that L.domEvent.removeListener() can be used later)
2. alter the CSS to gray out the control when the disable method is called and ensure the text <input> can't receive focus