title: Carrierwave check if remote file exists via fog
body: <p>I know I'm missing something really easy here...</p>
  
  <p><code>CarrierWave::Storage::Fog::File</code> has a method <a href="https://github.com/jnicklas/carrierwave/blob/master/lib/carrierwave/storage/fog.rb" rel="noreferrer">exists</a>?</p>
  
  <p>How do I use it?   I'm just trying to check whether a previously uploaded file exists on the remote storage.</p>
  
  <p>Neither my image object, nor my uploader object have access to this method.  Surely I don't have to create a new <code>CarrierWave::Storage::Fog::File</code> object to test if the file exists?</p>
  
  <p>If so, what parameters should I put in?   It takes the following: uploader, base, path
  (I used the uploader, store directory and image url, but that didn't work)</p>