title: assets_host in carrierwave config
body: <p>Since I am currently using nginx to serve public/uploads only in subdomain assets, and also I am using the client side template(eco) to render the image (so I cannot use image_tag or image_url helper method provided by rails), I need to set the model.image_url (which provided by carrierwave) to return url with same domain.</p>
  
  <p>Here is what I had tried: (in config/initializer/carrierwave.rb)</p>
  
  <pre><code>    CarrierWave.configuration do |config|
              config.assets_host = "http://assets.lvh.me:3000"
      end
  </code></pre>
  
  <p>But when I try this setting then the rails popup an error message:</p>
  
  <pre><code>    undefined method `assets_host=' for CarrierWave::Uploader::Base:Class (NoMethodError)
  </code></pre>
  
  <p>Since the README of carrierwave has this description of setting but only in fog section, so I'm wondering that is this function only work while using fog? Or did I miss anything?</p>
  
  <p>Thanks for help</p>