DefaultOpenDataset: add minimum x,y,w,h

Added min = 0 to the crop fields
TEMP: Pin to scifio SNAPSHOT

This is for the change to ImgSaver#saveImg which now returns Metadata.
DatasetService: improve javadoc of save methods
DatasetService: make save methods return Metadata

When SCIFIO writes a dataset, the relevant SCIFIO Writer constructs a
Metadata descriptor. It is nice to be able to access this descriptor
after the fact, in case the Writer recorded any details of the
transaction there. For example, the scifio-omero project introduces
an OMEROFormat whose Writer records the ID of the new OMERO Image.