Do not loose precision : keep a copy of original latlng

Currently, the last point of the animation is used for the final ``setLatLng()`` call on the marker.

This looses precision, especially if the map zoom level is low. To reproduce: bounce at zoom level 0, and zoom in to level 18: the marker position is wrong.

This patch keeps a reference to the original marker position and restores it at the end of the animation.

Since I didn't know if you were going to accept the PR https://github.com/maximeh/leaflet.bouncemarker/pull/2, I forked from your master: just tell me if you need any help for merging the two!