When a polyline is removed from the map, remove it from the internal polyline list.
Prevent exceptions when `that._map` is null

`that._map` is null after the first polyline is removed from the map, so
* Add safety code before dereferencing `that._map`
* For methods added to map, get map using `this` rather than `that._map`
Merge remote-tracking branch 'azavea/remove-polyline-safely'
Remove marker explicitly