Adding support for step batch import or background processing

I have a problem with importing large (18MB) XML file with approximately 11 000 of nodes to import. Xpathparser plugin process imports in one step, all nodes at once and this makes large load on server capacity.

I code a functionality to FeedsXPathParserBase.inc to handle import in steps (standard in 50 items limit). So now import with Xpath parser can run in background or be processed using batch operation.

Please test it on your site. Thanks