Use __dirname instead of 'cssify' to find the cssify package in the wrapper

Previously, cssify had to be installed at the "top-level" so that the `require()` added to each CSS file could find it. With this change, cssify can be installed as a dependency of some other module and still work correctly.

(My use-case is adding cssify as another transform to my Browserify-wrapping module, compilist: http://gigahard.mine.nu/hg/compilist/ )