Fix `fabric.Image.setElement`and image filter serialization. Closes #745

Fix `fabric.Image.setElement` - now _element and _originalElement (until now _originalImage) are set. Closes #745
Add additional parameter callback (invoked after filters habe been applied).
Fix `fabric.Image.fromObject` - don't overwrite image width/height with object width/height.
`fabric.Image.applyFilters` only resets _originalElement - fixes issue if object's width/height are different to _originaElement's width/height
[BACK_INCOMPAT] Renamed `fabric.Image.filters` method `toJSON` -> `toObject`
Fix serializiation of fabric.Image filter objects
Add image filter tests