The `options` argument for `ImageSpec` and `ProcessedImageField`
replaces `quality` and provides a more general solution, allowing the
user access to PIL's format-specific options (including "quality",
"progressive", and "optimize" for JPEGs).
At first, we only had a small amount of code for preserving
transparency in `img_to_fobj`, but as that code grew it became apparent
that it really didn't belong in the utility function. This commit
creates an `AutoConvert` processor which is automatically run before
saving unless you disable it by passing `autoconvert=False` to your
`ImageSpec` or `ProcessedImageField` constructor. `img_to_fobj` is once
again a simple utility function for creating a file object from a PIL
image.
Crop doesn't necessarily imply the any scaling is taking place. Several
ideas were discussed, from renaming Crop to combining both processors
into a single Resize processor (as they were in the original IK), but
those solutions were felt to either precluded future extension
(alternative resize modes) or make the API too verbose.
