Proposed support for Google Calendar

[martxw]

I've put together a branch of spring-social-google and spring-social-google-example to add in Google Calendar support, for a project I'm working on. They're my "add_calendar" branches.
I was wondering if you'd like me to submit a Pull Request?

My needs have mainly been read-only. As such, it does comprehensive calendar and event listing.
I also included simple event creation and deletion, and update for a subset of properties.

[GabiAxel]

Thanks for the initiative. You are welcome to submit a pull request. Please make sure to have documented code and at least basic tests as the rest of the project.