This also removes Pry.

Separate the UrlGenerator out from the Attachment. Some example plugins that could be written include generating thumbnails on the fly for different thumbnail sizes, or delaying the thumbnail generation until it is first called.

Fix a bug when passing a method name to `:if` and `:unless` option in `validates_attachment_presence`

Fixes #631

Fix 293, 601: STI subclassing overwrites options

ones (:private and :public_read) in the s3_test
completed test for lambda permission value

formulate assert conditions

bit of work

Closes #602