Make App.ImageFile use stream iterator

[Sidnei da Silva]

Here's a patch to make App.ImageFile to do two things:

 - Do *not* read the full file on startup, 64k is probably enough (too much?)
 - Return a filestream_iterator.

Should make Zope startup a little bit faster, and serving ZMI icons too.

[Sidnei da Silva (sidnei)]

Ops, forgot a print in there. :)

[Hanno Schlichting (hannosch)]

I applied a slightly modified version of the patch to Zope trunk. My version doesn't load the file at all, unless the mimetype cannot be guessed from the filename alone.