Implemented the @warn directive. A bit glitchy if you try to pass it a list with arithmetic expressions and interpolants, but it generally works without barfing.

Adding the 'append' builtin function because Bourbon needs it. Currently implemented in terms of 'join'; will need to rewrite.

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.