Fixed authentication using Linkedin

Fixed authentication using Linkedin as reported in https://github.com/gae-init/gae-init-auth/issues/34 based on example code provided at https://github.com/lepture/flask-oauthlib/blob/master/example/linkedin.py (though some further simplifications could be made based on existing code...)
Do it like in Yahoo! 
```
names = [response.get('firstName', ''), response.get('lastName', '')]
...
    name=' '.join(names).strip()

```
https://github.com/gae-init/gae-init-auth/blob/master/main/auth.py#L1000