remove preinstall hook that compiles stuff, we will use precompiled binaries to make it easier for people

make libsass as subtree, in the future do "git pull -s subtree libsass master" to pull in new changes

added callback support

Add support for node 0.6, 0.8

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.