Example:
  has_attached_file :image,
    :bucket => "testing",
    :path => ":attachment/:style/:basename.:extension"
    :s3_permissions => {
      :original => :private
    },
    :styles => {
      :thumb => '80x80>'
    }

The above will allow public read access on :thumb but require credentials for :original.

Example of setting default to private:
  has_attached_file :image,
    :bucket => "testing",
    :path => ":attachment/:style/:basename.:extension"
    :s3_permissions => :private
    :styles => {
      :thumb => '80x80>'
    }

The above will set all to private

Closes #398