Implemented the 'scale-color' built-in. Slight rounding discrepancies compared to Ruby Sass.

context.cpp

@@ -131,6 +131,7 @@ namespace Sass {
     register_function(fade_out_sig, fade_out);
     // Other Color Functions
     register_function(adjust_color_sig, adjust_color);
+    register_function(scale_color_sig, scale_color);
     register_function(change_color_sig, change_color);    
     // String Functions
     register_function(unquote_sig, unquote);

functions.hpp

@@ -147,6 +147,9 @@ namespace Sass {
     extern Signature adjust_color_sig;
     Node adjust_color(const Node, Environment&, Node_Factory&, string& path, size_t line);
 
+    extern Signature scale_color_sig;
+    Node scale_color(const Node, Environment&, Node_Factory&, string& path, size_t line);
+
     extern Signature change_color_sig;
     Node change_color(const Node, Environment&, Node_Factory&, string& path, size_t line);