Fixed #19 and improved work with multiple files

Now we can use one task for multiple css file, for example:

    dist: {
      cwd: 'app/assets/styles',
      src: [ "*.css" ],
      dest: "dist/assets/styles/",
    }

All css files from `app/assets/styles` will processed and saved to `dist/assets/styles` directory.

PS. Sorry for my bad english, I'm non-native speaker, feel free to correct me :)