change height and width options to be floats, and interpret values
between 0 and 1 to be percentages. For example, an options strings of
"0.5x" specifies 50% width and proportional height.
if both height and width are specified, we assume you want an image at
exactly those dimensions, so use imaging.Thumbnail, which will resize
and crop the image.
if either height or width are left unspecified, then it's just a simple
resize request.
- switch to disintegration/imaging library, which supports more of the
transformations I want to add
- pass image through untouched if provided Options are empty
