Leaks temp files when resizing

[James Westby]

When an icon has to be resized it is done with a NamedTemporaryFile(..., delete=False),
but the file is never cleaned up.

This leads to disks on production slowly filling up with icons.

In metadata.py _ensure_required_icon_size does

        new_path = convert_icon(icons[best_resolution], required_size)
        icons[required_size] = new_path

and that's the last reference to the icon.

The issues is that we're returning the paths to pkgme, but there's no way to
clean them up afterwards. This will probably require an extension to the interface.

Thanks,

James