code: def get_access_token(self, verifier=None):
          
          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)
docs: After user has authorized the request token, get access token
          with user supplied verifier.