Added support for DSpace collections

The DSpaceOutput generator now produces a collections file in the item
directory that contains a list of collections that the item should
belong to. Note that the Record, should contain a field named
"collections" in order for this file to be produced.
DSpaceOutputGenerator no longer prints empty fields
Fixed DSpaceOutputGenerator to not write empty values

DSpaceOutputGenerator was outputting empty values if they existed in the
field list. This commit fixes this.
New DSpace version