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.

(cherry picked from commit f58f73210621261c214136eb7514be459032ee0f)

(cherry picked from commit 43946120a6231be06d59c8f28969946c213e6872)

Fix broken validates_attachment_size error message. The min and max values were not being passed to attachment_definitions and thus were not available for :min and :max string replacement when the error message was built, leading to spaces rather than byte values for :min and :max.
(cherry picked from commit 3fdb1160b272e71653d18d0b5945073e615c0a2d)

(cherry picked from commit 6a0ff6f0f76a92a25c9cbbc315d3d7d11d1d7720)