Implemented adjust-color -- what was once the most troublesome is now relatively simple thanks to the improved built-in argument specification and binding mechanism!

Trying out new scheme built-in function definitions. Allows built-ins to have default args and whatnot, and to be specified more easily than before.

Bam, re-did argument binding. More robust, and allows default arg values to refer to previoius args.

Forcing required params to precede optional params in function and mixin definitions, to match recent Sass changes.

Conflicts:
	functions.cpp