Fix minimagick rotate and add watermark

I fixed the minimagick rotate and added watermarking for minimagick.
Sorry no specs.
Need to refactor this so that it works for both RMagick and MiniMagick

for example: 

```ruby
module CarrierWave
  module Processing
    module RMagick
      def rotate
        manipulate! do |img|
          img = img.auto_orient
          img = yield(img) if block_given?
          img
        end
      end
    end
  end
end

module CarrierWave
  module Processing
    module MiniMagick
      def rotate
        manipulate! do |img|
          img.auto_orient
          img = yield(img) if block_given?
          img
        end
      end
    end
  end
end
```