Crop-preview with aspect ratio keeping support (optional)

Add possibility to make crop-preview not aspect ratio fixed. Currently images are forced to scaled in preview loosing selection aspect ratio. If service accepts images with different ratio, we need to give user access to this option. After merging this request it will be possible. To do it `aspectRatio` option must be removed from options in `$('...').cropper()`, and `keepAspectRatio: true` option must be added in `$('...').fileapi({elements:{preview:{}}})` option.