New Dropzone.options.thumbnailKeepAspectRatio option

This pull implements a new boolean `Dropzone.options.thumbnailKeepAspectRatio` option to allow thumbs keep their original aspect ratio based `thumbnailWidth` and `thumbnailHeight` values. 

To make horizontal and vertical previews reasonable similar it considers the longer thumbnail value to use on its longest side.

This also requires to remove fixed styling and remove `position: absolute` from `.details img`.