Prevent the same header being used each time

- Stop using `grunt.util._` in favor of `lodash`. The former has been
  deprecated for sometime and is super old. The latter is awesome :)
- Make use of `lodash` to clone headers before uploading any files.
  This ensures you don't end up with the mimetype of the first file you
  upload for all your files. This is only an issue if you're using the
  undocumented `headers` option. More discussion can be found at
  rtgibbons/grunt-cloudfiles#21.

No tests cause that's how we're rolling