Add -remaining flag to estimate remaining time needed to finish the import

I've added a remaining to flag to indicate how much time is needed to finish the import. It loads the whole XML into a SimpleXMLElement in the beginning and counts the objects according to the path in the <aoe_import> setting in the module's configuration. This obviously increases the initial start up time, that is why I made it a separate flag.

Log line before;
==> (1) - [--- Processed //ProductExport[1]/Products[1]/Product[1] using processor 'Elgentos_Vendit_Model_ImportProducts' ---]

Log line after;
==> (1 / 2998 - estimated time left; 04:21:07) - [--- Processed //ProductExport[1]/Products[1]/Product[1] using processor 'Elgentos_Vendit_Model_ImportProducts' ---]