This function deals with the Webfinger lookup, discovering a connecting
user's storage details.

@param {string} userAddress - user@host

@returns {Promise} A promise for an object with the following properties.
href - Storage base URL,
storageApi - RS protocol version,
authUrl - OAuth URL,
properties - Webfinger link properties
function Discover(userAddress) {
  return new Promise((resolve, reject) => {

    if (userAddress in cachedInfo) {
      return resolve(cachedInfo[userAddress]);
    }

    var webFinger = new WebFinger({
      tls_only: false,
      uri_fallback: true,
      request_timeout: 5000
    });

    return webFinger.lookup(userAddress, function (err, response) {
      if (err) {
        return reject(err);
      } else if ((typeof response.idx.links.remotestorage !== 'object') ||
                 (typeof response.idx.links.remotestorage.length !== 'number') ||
                 (response.idx.links.remotestorage.length <= 0)) {
        log("[Discover] WebFinger record for " + userAddress + " does not have remotestorage defined in the links section ", JSON.stringify(response.json));
        return reject("WebFinger record for " + userAddress + " does not have remotestorage defined in the links section.");
      }

      var rs = response.idx.links.remotestorage[0];
      var authURL = rs.properties['http://tools.ietf.org/html/rfc6749#section-4.2'] ||
                    rs.properties['auth-endpoint'];
      var storageApi = rs.properties['http://remotestorage.io/spec/version'] ||
                       rs.type;

      // cache fetched data
      cachedInfo[userAddress] = {
        href: rs.href,
        storageApi: storageApi,
        authURL: authURL,
        properties: rs.properties
      };

      if (hasLocalStorage) {
        localStorage[SETTINGS_KEY] = JSON.stringify({ cache: cachedInfo });
      }

      return resolve(cachedInfo[userAddress]);
    });
  });
}