Re-structure into strategies

This pull request also makes including RetinaRails::CarrierWave obsolete. It just works out of the box. Probably a good idea to use this for Paperclip aswell. You can now also define non-retina images;

```ruby
class ExampleUploader < CarrierWave::Uploader::Base

  retina!

  version :small do
    process :resize_to_fill => [30, 30]
    process :retina_quality => 25
  end

  version :large, :retina => false do
    process :resize_to_fill => [1000, 1000]
  end

end
```