prevent false move events

On a window resize, move and moveend events are fired even if the map
did not move, for instance, maximizing a window that contains a map with
a fixed size. This may create an unnecessary expense for an application
depending on how it handles these events. L.Map.invalidateSize
calculates the move offset and proceeds with pan and move events, even
when the offset is (0, 0). I added a conditional block to prevent this.
Merge pull request #1 from oslek/false-move-events

prevent false move events