allow mount uploader to serialized field

Sometimes we need to mount uploader to serialized field, for example if we do not want to create additional table to handle dynamic uploader. So if we specify `serialize_to` options for `mount_uploader` command it will automatically add necessary logic
```ruby
class User < ActiveRecord::Base
  serialize :documents, Hash
  mount_uploader :document1, DocumentUploader, serialize_to: :documents
end
```

See [this link](http://stackoverflow.com/questions/9016751/carrierwave-mount-uploader-on-serialized-dynamic-attribute).