Xing provider (& Foursquare Provider)

Here is a new Oauth1 Provider for XING, as well as the Foursquare Provider (again - last time the pull request was for the wrong branch).

![Screen Shot 2013-01-18 at 10 01 22](https://f.cloud.github.com/assets/1033269/77600/b9c9c2c2-614d-11e2-866b-0152c0a13097.png)