Setting storage class does not support deconstructors

[michjnich]

When setting the storage, sorl does not allow for a class instance being deconstructible, which is necessary to be able to use backends with arguments.

sorl.thumbnails.images.py

cls = self.storage._wrapped.__class__

For a deconstructible class this can use self.storage.deconstruct (checking whether the class is deconstructible first to ensure backwards compatibility).

deserialize could check whether the storage is a list (deconstructible) or a single value (old method)