remove unused includes
mapnik::util::variant in mapnik::value
Merge branch 'master' into custom-variant-2

Conflicts:
	include/mapnik/value.hpp
replace boost::spirit::karma alternate operator (uses boost::variant) with direct util::variant to string conversion
value traits to control to value conversions + fix is_null
use is_null_visitor
add link to custom generator TODO
transform : is_null_node visitor
topojson - revert to using boost::variant for mapnik::topojson::geometry (TODO)
Merge branch 'master' into custom-variant-2

Conflicts:
	include/mapnik/value.hpp
add getter interface compatible with boost::variant
expose cairo_renderer<cairo_surface_ptr>
convert symbolizer properties to use util::variant
fix cairo_renderer usage in demo/viewer + revert aac69d0b34203f856070cfe5bf909fbf84853b7f
AGG/Cairo - fix pattern opacity
make mapnik::symbolizer a mapnik::util::variant<Symbolizers...>
Merge branch 'master' into custom-variant-2
fix typo - uncomment column_collector
convert all boost::variant to util::variant
apart from topojson (TODO)
cleanup