Add source attribute for FileObj (#51)

The `source` attribute should be added to FileObj so it can be
used to construct valid JSON objects for the ingest move API.

Signed-off-by: David Brown <dmlb2000@gmail.com>

pacifica/uploader/metadata/metadata.py

@@ -138,6 +138,7 @@ class MetaObj(_MetaObj):
 FILE_KEYS = [
     'destinationTable',
     'name',
+    'source',
     'subdir',
     'size',
     'hashtype',