POM: pin to scifio 0.13.0 release

This is for the change to ImgSaver#saveImg which now returns Metadata.
POM: bump the major version (0.6.0-SNAPSHOT)

This bump is due to an impending API change to DatasetService.
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.
Merge branch 'return-saver-metadata'

This branch makes DatasetService#save return the SCIFIO Metadata object
associated with the save operation.