Create module pgmapcss.colors; include in function 'match'

- functions: to_color(), color_values()
- include in function via resource_string()
- eval: deluma() and eval_lighten() use function color_values()
Exception 'CompileError'
Compile function 'match': new function: warning()
Compile Value: Move type specific compiling to pgmapcss.types
pgmapcss.types: add type color; specify type color in mapnik mapcss
Rename warning() to debug()
Improve inclusion of color functions
Modules may register includes
Includes: add function includes()
Accept colors in short notation, e.g. '#f00'
Replace type base by type default
stat_property_values(): as property's type stat_value()
Types: new type 'values'; move allowed values from stat to type
Types: remove unused code
New canvas property: buffer
Process Mapnik: For the mapnik SQL query only add columns from hstore as needed
Type: 'pixels' (width, offset, ...)
Types::icon: needs module os