Add Rubocop, do some stylistic cleanup
Cleaner description
Require current versions of Paperclip, image_optim for 2.0
Disable all compressors by default, ignore missing binaries

image_optim defaults to enabling every supported binary, and therefor
requiring its presence. We skip_missing_workers, so that when image_optim
starts supporting a new one which we do not yet disable, uploads do not
start breaking due to the missing compression binary.
Support mergeable global, per-model and per-style options
Rails generator for a global config initializer
Add a post-install message with information about the 2.0 transition
Update Rails dependency