Allow user-defined geometries in image#thumbnail_dimensions

Hey there,

The `image_fu` helper (and `thumbnail_dimensions` method) supported the following scenarios:

* `image_fu(image, nil)`
* `image_fu(image, "200x200")`
* `image_fu(image, "200x200#")`, etc.

With this PR now you can pass user-defined sizes (from Refinery::Images.user_image_sizes)

Example:
`image_fu(image, :medium)`