'Add and remove an callback.'

def test_del(self):

    self.handler.add_callback(self.exit, 'traverse', 'pass', author=self.char1, valid=True)
    self.call(CmdCallback(), '/del    out    =    traverse    1', caller=self.char2)
    self.assertTrue((len(self.handler.get_callbacks(self.exit).get('traverse', [])) == 1))
    self.call(CmdCallback(), '/del    out    =    traverse    1')
    self.assertTrue((len(self.handler.get_callbacks(self.exit).get('traverse', [])) == 0))
