Fixes #743

Fixes #745

In the effort of decoupling Rails from unit specs, you can't have any code that calls `Rails` (since it isn't available). In my unit specs that need Paperclip, I call `Paperclip::Railtie.insert` and all is good :)

Introducing `:restricted_characters` in Paperclip::Attachment.default_options  so people can override their blacklist characters by override that setting.

Fixes #703

Closes #678. Closes #450.

If a model has an existing attachment and styles, and later a new attachment is added (not just new styles), "rake paperclip:refresh:missing_styles" would crash with "TypeError: can't convert nil into Array".

#fingerprint now consistently returns nil before and after saving an attachment if the value can't be written to the db.

From https://github.com/pcreux/paperclip/commit/52374958ef83f1641cf008aead27c23b25fb842d

Conflicts:

	lib/paperclip/attachment.rb