Added s3 support. Fixes #1

Adds s3 storage for digital goods.

Files are uploaded to s3 as private and are still mediated via the digitals controller. For s3 downloads, the digitals controller redirects the user to an expiring download link. Added a new preference ````Spree::DigitalConfiguration[:s3_expiration_seconds]```` which determines the expiration time of the s3 download link. Added an "attachment" content disposition to the s3 header so goods are downloaded rather than displayed, although this might need to be a preference.

I also updated some of the 2-0-stable overrides which weren't properly updated for Spree 2.0
Unfortunately, I've only tested against 2-0-stable and haven't added any test coverage yet.