Returns a Premier account signed url.

def get_signed_url(self, params):
        '''Returns a Premier account signed url.'''
        params['client'] = self.client_id
        url_params = {'protocol': self.protocol, 'domain': self.domain,
                      'service': self.service, 'params': urlencode(params)}
                      
        secret = base64.urlsafe_b64decode(self.secret_key)
        url_params['url_part'] = (
            '/maps/api/%(service)s/json?%(params)s' % url_params)
        signature = hmac.new(secret, url_params['url_part'], hashlib.sha1)
        url_params['signature'] = base64.urlsafe_b64encode(signature.digest())

        return ('%(protocol)s://%(domain)s%(url_part)s'
                '&signature=%(signature)s' % url_params)