Support adding to map after map init

This change supports adding the control to a map via "map.addControl(
L.Control.loading() );" as an alternative method along side the already
supported map options method.
(Includes the concept of hashed loading states.)
Merge remote-tracking branch 'origin/master' into postdefacto-addable
Move map attachment

In the context of ex post facto control addition, it makes much more
sense to attach the control to the map in the onAdd function.
Merge branch 'expostfacto-addable' of git://github.com/codefox421/Leaflet.loading into codefox421-expostfacto-addable
Consolidate layer event logic