Remove Node Requirement enabling a simple import to drupal as asset

Hi,

My goal was upload videos and then associate them on FileField Sources module, and file import was perfect, except for the fact that you are not allow to upload files without associate them with a node, for this reason I create this patch, which basically allows the file exists before the node. Thus a node null is create it if you leave the field "Target node ID" empty.

Please consider this change on next release.

Jason.