cleanup of mapcss implementation:
* added internal states as pseudoclasses and renamed them (:active, :(un)tagged)
* symbol_shape, symbol_size instead of circle_radius
* node selector also matches geojson Points (e.g. for placeholders)
* etc.
fixed mapcss "set" syntax (InstructionStyle)
support for "alias" mapcss properties (like casing-color instead of color)
* casing-*
* symbol-stroke-*
* symbol-fill-*
wrong link in attribution
fixed bug preventing black color to be rendered