Adding infrastructure for pure Sass functions. Necessary to pass more contextual data around during parsing, since the set of expressions permitted in functions is relatively limited.

Bam, fixed the environment handling with a 2-line change. Added an option to apply_mixin to allow for dynamic scope.

Using a multimap to store selector extension requests -- necessary to allow a selector to be extended multiple times.

Fixing a portability problem -- apparently, older versions of C++ don't automatically zero out objects in their default constructors.

Conflicts:
	eval_apply.cpp
	node.hpp