code: def get_login_button_url(self, button_color=None, caption_color=None, button_size=None):
          
          if not button_color in settings.LOGIN_BUTTON_COLORS:
              button_color = settings.LOGIN_BUTTON_COLORS[0]
          if not caption_color in settings.LOGIN_BUTTON_CAPTION_COLORS:
              caption_color = settings.LOGIN_BUTTON_CAPTION_COLORS[0]
          if settings.LOGIN_BUTTON_SIZES.has_key(button_size):
              button_size = settings.LOGIN_BUTTON_SIZES[button_size]
          else:
              button_size = settings.LOGIN_BUTTON_SIZES['None']
          return settings.LOGIN_BUTTON_URL % (button_color, 
                                              caption_color, 
                                              button_size)
docs: Return URL for image used for RunKeeper Login button.
          
          @param button_color:  Button color. Either 'blue', 'grey' or 'black'.
                                Default: 'blue'.
          @param caption_color: Button text color. Either 'white' or 'black'.
                                Default: 'white'
          @param button_size:   Button width in pixels. Either 200, 300 or 600.
                                Default: 200
          @return:              URL for Login Button Image.