Some ItemImport clean up

Hi @peterdietz,

I have done some cleanup in ItemImport class which is actually the merging of two almost similar methods that handle UI requests for batch import.

Apart from that, I have done some other changes regarding the unzip of files:

1) I pass as an argument in the zip method the path that files should be extracted. I keep backward compatibility by declaring the old method again and passing null to the new parameter.
 
2) I close the unzip file at the end of the unzip method, since I noticed that I could not delete the batci import folder at the end or at user request from the UI.

Regards,

Kostas