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

This adds functionality to add additional params to the querystring for
S3 expiring URLs. The reason for this is if you want to override
response_content_type or response_content_disposition with expiring
URLs, you have to change Amazon's signature, which gets signed before
adding additional options to the querystring. Because it's added later,
and because Amazon's signature includes the full URL, the signature is
bad and the request fails.

To use this feature:

    has_attached_file :avatar,
                      :s3_url_options => { :response_content_disposition => "inline" }

Additionally, you can pass a lambda and it'll be evaluated when the URL
is generated. If, for example, the content type is incorrect in Amazon
(either empty or application/octet-stream), you can effectively cast the
response from Amazon as a particular content type.

    has_attached_file :avatar,
                      :s3_url_options => lambda {|model| { :response_content_type => model.avatar_content_type } }

We've deprecate these method for a while, it's time to remove it.

Fixes #723

Fixes #646