Replace node-sassy with node-sass (eliminate Ruby requirement)

[xdissent]

This PR uses node-sass, which is a wrapper for the libsass C implementation. This gets rid of the Ruby requirement, at the cost of .sass-file processing. In that regard it's kind of an opinionated PR, so I understand if you're not interested.

[techpines]

I'm cool with this. The sass module was community generated. The only caveat here, is that I don't want to make node-sass a dependency in the package.json.

Binary dependencies sometimes make installing more of a pain, so I would just make a note that you need to do npm install node-sass to use the module. Otherwise installing asset-rack might fail for people that aren't even using it for sass, like me :)

I'll let this sit for a bit to see if anyone that uses the Sass module has an issue with it. Otherwise, I'm fine with merging it if the api is the same.

[xdissent]

Now node-sass is in optionalDependencies, meaning npm will carry on if installation fails for any reason. I put node-sassy back in dependencies since it's tiny and doesn't hurt anything if it's installed and you don't use it. Sass assets will detect whether node-sass is available.

[freshlogic]

Would love to see this merged