Make sure zipfiles which contain directory entries work

When Plomino creates a zipfile, it doesn't create directory entries such
as the `resources/` entry below. However, unless you pass the `-D`
option, `zip` does:

Archive:  /home/jean/tmp/historydb.zip
  Length      Date    Time    Name
---------  ---------- -----   ----
     1738  2013-10-10 20:32   dbsettings.xml
    21266  2013-11-12 11:41   frmHistory.xml
        0  2013-10-11 18:02   resources/
     1183  2013-10-11 18:02   resources/libConfig.xml
     5790  2013-10-11 18:02   subAccess.xml
     6182  2013-10-11 18:02   subChildFormFields.xml
---------                     -------
    36159                     6 files

That directory failed like this:

2013-11-13 19:17:26 ERROR Zope.SiteErrorLog 1384345046.850.594614657075 http://local:8080/Plone/testdb/importDesign
Traceback (innermost last):
  Module ZPublisher.Publish, line 138, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 48, in call_object
  Module Products.CMFPlomino.PlominoDesignManager, line 559, in importDesign
  Module Products.CMFPlomino.PlominoDesignManager, line 1228, in importDesignFromZip
  Module xml.dom.minidom, line 1931, in parseString
  Module xml.dom.expatbuilder, line 940, in parseString
  Module xml.dom.expatbuilder, line 223, in parseString
ExpatError: no element found: line 1, column 0
Merge pull request #480 from plomino/jean-fix-zipfile-import

Make sure zipfiles which contain directory entries work

Tests pass and the change is relatively trivial, so I'll merge.