Integrate xing properly

[ellenkoenig]

If needed, user env property to configure the callback Url?

[ellenkoenig]

http://www.heise.de/ix/artikel/Sozial-integriert-1892413.html

[ellenkoenig]

or: http://blog.apicultur.com/en/using-the-xing-api-to-migrate-your-linkedin-contacts/

[ellenkoenig]

Should also replace the currently hardcoded authentication data (secret etc.) by env property