Fixed name collision with boost::graph.
Fixed name collision with boost::graph.

Renamed edge_color() to get_edge_color() to resolve following error:
In file included from /home/kaben/Developer/Source/openvoronoi-cpp-api/cpp_examples/basic_offset/main.cpp:6:
/usr/include/boost/graph/properties.hpp: In function ‘void write_edge_to_svd(ovd::HEGraph&, svg::Document&, ovd::HEEdge)’:
/usr/include/boost/graph/properties.hpp:99: error: ‘boost::edge_color’ is not a function,
/usr/local/include/openvoronoi/utility/vd2svg.hpp:19: error:   conflict with ‘svg::Color edge_color(ovd::HEGraph&, ovd::HEEdge)’
/usr/local/include/openvoronoi/utility/vd2svg.hpp:33: error:   in call to ‘edge_color’

aewallin/openvoronoi,aewallin/openvoronoi,aewallin/openvoronoi