Remove pry-debugger reference
Make StoryAttachmentsController better able to handle XHR

See crowdcover/crowdcover#214
Fix the before_post_process method

See crowdcover/crowdcover#214

The previous `#image?` implementation was incorrectly telling the
`Paperclip::Attachment` to stop post processing. The method is now
correctly matching image content types.
Override `#serializable_hash` to customize JSON/XML rendering

See crowdcover/crowdcover#214

JSON and XML representations of `StoryAttachment` instances care about
image urls and sizes, not the fields Paperclip uses to determine image
information.
Add attachments array to the `.default_params`

See crowdcover/crowdcover#214

The `StoryAttachment` ids need a place to go for a given section.