Major improvements

- Ensure we are always in the same UI-session when calling the
TokenCallbackServiceHandler. Therefore all HTTP requests from JavaScript
to the ServiceHandler contain the request property "cid". 
- new sign-in button variant using client scripting (signInButton.js)
- add demo how to correctly aquire a refresh token in the
TokenCallbackServiceHandler
- add dependency: Google+ client API:
  - https://developers.google.com/resources/api-libraries/download/plus/v1/java 
  - v1-rev118-1.17.0-rc