After user has authorized the request token, get access token
        with user supplied verifier.

def get_access_token(self, verifier=None):
        """
        After user has authorized the request token, get access token
        with user supplied verifier.
        """
        try:
            url = self._get_oauth_url('access_token')

            #build request
            query = {
                'client_id': self._client_id,
                'client_secret': self._client_secret,
                'grant_type': 'authorization_code',
                'redirect_uri': self.callback,
                'code': str(verifier)
            }

            query_str = self.urlencode(query)
            request = url + '?' + query_str

            #send request
            resp = urllib.urlopen(request)
            json = simplejson.loads(resp.read())

            self.access_token = json['access_token']

            return self.access_token

        except Exception, e:
            raise FoursquareError(e)