waiting for item to be created before modifying metadata

to avoid `item.modify_metadata()` raise error (like `Unable to locate identifier`)

uploader.py

@@ -248,6 +248,14 @@ def upload(wikis, config={}, uploadeddumps=[]):
             #Upload files and update metadata
             try:
                 item.upload(dumpdir + '/' + dump, metadata=md, access_key=accesskey, secret_key=secretkey, verbose=True, queue_derive=False)
+
+                retry = 20
+                while not item.exists and retry > 0:
+                    retry -= 1
+                    print "Waitting for item \"%s\" to be created... (%s)" % (identifier, retry)
+                    time.sleep(10)
+                    item = get_item(identifier)
+
                 item.modify_metadata(md) # update
                 print 'You can find it in https://archive.org/details/%s' % (identifier)
                 uploadeddumps.append(dump)