Use minimagick for transformations

[timriley]

• Devise a way to encode a series of minimagick operations/arguments into a string
• For security, see if we can incorporate a pre-shared key into this encoding (i.e. the attache secret key, which is used to verify all the uploads to attache already)
• Create a decoder for the above
• Create an object to take the decoded steps and apply them to an image
• Rip out Paperclip::Thumbnail from Attache::ResizeJob and replace with minimagick operations
• Rename ResizeJob to ProcessJob or something more generic
• Handle "original" geometries somehow
• Remove geometry_whitelist (not needed with HMAC verification)
• Add performance optimisation back for large images