Implement the carrierwave default_url method in uploader.

This change provides a means for accessing the fog file url prior to model persistence.

We came across an issue where we'd uploaded the file and then attempted to save the key. In situations where the key is *not* saved due to validation errors, there was no way (that I could find) to generate the full path to the uploaded file.  This PR addresses that by using carrierwave's `default_url` hook.

Certainly open to any/all feedback.