Get the charm icon.

        @param charm_id The ID of the charm.
        @param channel Optional channel name.

def charm_icon(self, charm_id, channel=None):
        '''Get the charm icon.

        @param charm_id The ID of the charm.
        @param channel Optional channel name.
        '''
        url = self.charm_icon_url(charm_id, channel=channel)
        response = self._get(url)
        return response.content