Moved AR include into a configure method to prevent non-inclusion when using the git repo in the Gemfile

Rails.root returns a Pathname and you can now declare your attachment
without having to call #to_s first:

has_attached_file :avatar,
                  :storage => :s3,
                  :s3_credentials => Rails.root.join('s3.yml')
(cherry picked from commit 65d7600b7246cb3bd8f10805fb618e5839694318)

Added a rake task for testing all versions. Changed gem statements to use ~> instead of hard versions.

These callbacks are different from pre-rails 3. They do NOT cancel the
after_ if the before_ returns false. They will cancell inner callbacks,
however, but that's it. This is a change in functionality.

Determine content_type with file command's --mime-type when content_type is not found in the default cases.
(cherry picked from commit 2ca52df1)

styles encapsulated, to delay calling of procs until context is useful and to allow per-style processor lists

(cherry picked from commit 61dbcb3e36c0bc9742d045dd1e54f846043a1569)

Fixed an error where ImageMagick wasn't identifying images because the Tempfiles did not have the right extension.