Example of setting `cache_dir` outside of project folder 

For now `cache_dir` is independent of `store_dir`.

I am not sure if it would be good to inherit the `cache_dir` path from `store_dir`, but if i set `store_dir` to `/tmp/projectname` then `Carrierwave` keeps creating a `spec/public/uploads/tmp` folder when running tests of a rails app.

spent some time trying to figure out why until found that `cache_dir` exists as a param.

thanks