LP 1672519: Make marc_export descendants aware.

LP 1672519: Make marc_export descendants aware.

Add a --descendants option to marc_export to get records for an
org. unit and its descendants.

To test
-------
[1] Run marc_export with -i --library FOO. Verify that the resulting
    records have 852 fields only from FOO.
[2] Run marc_export with -i --descendants FOO. Verify that the resulting
    records have 852 fields from FOO *and* all descendants of FOO.
[3] Run marc_export using various combinations of --library and --descendants;
    verify that items from the expected libraries show up as 852 fields.

Signed-off-by: Jason Stephenson <jason@sigio.com>
Signed-off-by: Rogan Hamby <rogan.hamby@gmail.com>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>