Edit/delete handler bug.

First commit:
When in edit/delete mode, revertLayers has to be called when another handler is clicked on.

To reproduce in http://leaflet.github.io/Leaflet.draw/
-Draw two markers
-Delete one but do not click save/cancel
-Click edit button.

The marker is deleted even though the change was not saved.

Second commit:
-draw:deletestart and draw:deletestop are fired in delete handler instead of draw:editstart and draw:editstop.
-enable and disable functions are reshaped so that a handler cannot be activated while the other is still active.

This is useful when drawLayer has to be updated in some way when entering delete mode and in another way when entering edit mode.