Server side example

@Scarygami Adding a server side component. I think some more clean up can be done and documentation of methods. Give it a spin and let me know what you think?

```
$ pub install
$ dart bin/server.dart
```

The server now provides the `/people` and `/disconnect` methods. So after the client has authenticated we shoot the `code` to the server and let the server get `access_token` using the `postmessage` redirect uri. This is similar to what the java example does in https://github.com/googleplus/gplus-quickstart-java