Fixes #786

Needs work for S3 Attachments.

Fix AttachmentContentTypeValidator to work with multiple values

Current validation check in AttachmentContentTypeValidator is simply wrong, it will add an error if any of the allowed_types fails comparison with value, so it will pass only if value is either empty or equals to each and every one of allowed_types.

Add test to check that validation passes if even one of content types match.

Rails 3.0.x doesn't support new migration style

This reverts commit b7bfdec3.

Note: find_each doesn't support order because it already specifies it.
And it happens to be 'id' as well.

minor typo in explanatory rdoc-comment

The new default :path and :include include the name of the model and
also nests the model ID under a series of subdirectories, improving
filesystem access speed when more than 1024 models have saved
attachments.

The easiest way to upgrade is to add an explicit :url and :path to your
has_attached_file calls:

    has_attached_file :avatar,
      :path => ":rails_root/public/system/:attachment/:id/:style/:filename",
      :url => "/system/:attachment/:id/:style/:filename"

Fixes #738