code: def get_grades(self, site):
          
          tools = self.get_tools(site)
          grade_tool_filter = [x.href for x in tools if x.name == 'gradebook-tool']
          if not grade_tool_filter:
              return []
          response = self._session.get(grade_tool_filter[0])
          response.raise_for_status()
          iframes = self._html_iface.get_iframes(response.text)
          iframe_url = ''
          for frame in iframes:
              if frame['title'] == 'Gradebook ':
                  iframe_url = frame['src']
          if iframe_url == '':
              print "WARNING: NO GRADEBOOK IFRAMES FOUND"
          response = self._session.get(iframe_url)
          response.raise_for_status()
          grade_dict_list = self._html_iface.get_grades(response.text)
          return grade_dict_list
docs: Gets a list of grades associated with a site. The return type is a dictionary
          whose keys are assignment categories, similar to how the page is laid out
          in TSquare.