Fix bundler dependency resolution during development

This allows bundler to complete gem dependency resolution during
development. This is only relevant for paperclip developers. Please see:
https://github.com/thoughtbot/paperclip/issues/1256 and thanks to
@nkondratyev
Added exception handling to paperclip:refresh:thumbnails rake task.

https://github.com/thoughtbot/paperclip/issues/760
Remove the InstanceMethods and reduce class-level state
wip: Paperclip::Tasks::Attachments
Push attachment definitions into a list of attachments for the rake task
Rid ourselves of attachment_definitions
Pull the file setter into a separate object
Move the attachment getter to the HasAttachedFile class
Move the attachment query to the HasAttachedFile class
Move validates_each into HasAttachedFile class
Remove classes_with_attachments
Move the check for path collision out
Clear attachment styles before rake task tests.
Remove redundant Hash subclass.
Move Tasks registration to HasAttachedFile.
Move after_save callback to HasAttachedFile
(wip) Fix failing cucumber spec

There was a spec that was failing because we removed .attachment_definitions
from models, I expected this to fix it, but apparently it hasn't.
Clear attachments in a setup
Fix a typo
Path collision checks are gone