Make compatible with Leaflet master

The order of events during layer initialization was changed in
Leaflet/Leaflet@bd0bc4f2e56f3ccfe8d2817e2a213b0e1480ee26, resulting
in _onMiniMapMoving being called before _lastAimingRectPosition
has been initialized.
Merge pull request #23 from jfirebaugh/leaflet-master

Make compatible with Leaflet master