Initiated S4 class: SpatialPolygonsTiles
Added printouts for current zoom level
during worker and recurrent functions.
Most of the necessary changes made to switch to efficient
updating. Creates SpatialPolygonsTiles objects and now
has a separate plotting function.
Fixed projection bug - now working properly.
Added ... argument to createTiles in order to allow for additional plot arguments.
Added subset function for SpatialPolygonsTiles object.
Fixed code to allow vectors of plotting parameters,
such as colour and border to be passed correctly to plot function.