Use S3 to store images
Add gems for carrierwave_backgrounder.
Configure carrierwave for S3.
Initialize carrierwave_backgrounder & delayed_job.
Question#image_in_base64 returns the cached image if it exists, otherwise the S3 image.
Save images to S3 with delayed_job.
Don't send Question#image_tmp from backbone.
Question#image_url returns the cached image if it exists, otherwise the S3 version.
Set the `cache_dir` for the ImageUploader.
Use `question.image_url` instead of `question.image.url`.
Question#image_url returns the URL to the image. Not the absolute path.
If the cached image is displayed for a question, show it at the right size.
Move specs for Question's images from question_spec to shared_examples_for_question.
Mock Carrierwave (and Fog) properly so all specs pass.