Bundle css files with compiled stylus.

Instead of calling `css()` from the layout multiple times, you can just include the `//= require` directives in stylus. This way, in production, everything gets all bundled up nice and tidy.

Also, csso and uglify-js are required for connect-assets@3.0.0-beta1 in production. I think I might add those as dependencies before v3 gets shipped… but until then, you’ll have to include them.

Also, apparently csso doesn’t like compiling empty files. Since connect-assets precompiles everything in your assets tree by default in production, variables.styl ends up throwing an error. You could specify a precompilation list to connect-assets, but I just added a style rule to variables.styl instead. Let me know if you’d like me to change it.