index.html
added a two config-lines that can simply be uncommented for activating the "twilio-mode" or the "php-credentials-mode"
turn-credentials.php (new file)
will generate username and password for a turn server with a static-auth-secret and will offer it in json-format
main.js
added a section that requests output from php-credentials.php and adds the username, password, stun-server and turn-server into the configuration
