Override urllib2 default user-agent

        Open a file accross an http server

        '''

        try:

            self.fo = urllib2.urlopen(path, timeout=self.timeout)

            headers = {"User-Agent": "%s v%s" % (installsystems.canonical_name,

                                                 installsystems.version)}

            request = urllib2.Request(path, None, headers)

            self.fo = urllib2.urlopen(request, timeout=self.timeout)

        except Exception as e:

            raise ISError("Unable to open %s" % path, e)

        # get file size