[snippet] implement "remember me" with BrowserId

auth.js

// https://github.com/mozilla/browserid/wiki/Advanced-BrowserID-Features

function login () {
  function gotAssertion (assertion) {
    if (assertion) {
      // User chose not to log in, or an error occurred
    } else {
      // Ajax call to server to check email
    }
  }

  // Retrieve previous session
  navigator.id.get(function (assertion) {
    if (assertion) {
      // Got one!
      gotAssertion(assertion);
    } else {
      // Need to log in for the first time
      navigator.id.get(gotAssertion, {allowPersistent: true});
    }
  }, {silent: true});
}

function logout() {
  navigator.id.logout(function (ok) {
    // ok if the user is properly logged out
  });
}