Add method to retrieve all attachments

In order to get style URLs that could be served statically rather than
generating URLs Rails must process it is necessary to have more
information than the attachment id when rendering a story page. The
`attachments` method retrieves all the `StoryAttachment` instances whose
ids are part of this `Story`'s section.
Expose attachments to `story.md.erb`

Enable `story.md.erb` to access a full `StoryAttachment` reference by
using the `StoryAttachment#id` as a key to a `Hash`.
Add attachment_url to application.yml

The `ATTACHMENT_URL` configuration value will identify the domain and
protocol from which `StoryAttachment` assets will be loaded.
Use the attachment id as a string for the key

The attachment ids come throught the `StoryController` as Strings so it
is easier during render to use a String as the lookup key.
Use file path based attachment URL

Changing to file path based attachment URLs allows for the attachments
folder to served by a proxy or CDN with relative ease.
Merge pull request #3 from bryanjswift/attachment-urls

Modify how attachments are saved in static story