Add field for attachment_url preference to image settings page

Fixes #2344

Conflicts:

	core/app/views/spree/admin/image_settings/edit.html.erb
Remove commented out lines from payment factory
Bump ActiveMerchant dependency.

Fixes #2346
Depend on Ransack 0.7.2

This is due to ernie/ransack#176.
Use localized error message for email validator

Fixes #2364
Fixed issue with duplicate thumbnails on ProductController#show

Fixes #2361