Osng 487

This pull request contains the (undocumentet) interface for getting all extension-meta-informations from the osiam server. This interface is needed by addon-administration!
I thought about this, and I was wrong. You can remove this, if you create a service layer which is just for the extension stuff, maybe an [ExtensionService](https://gist.github.com/dacrome/4f800b7f10fa7a4969f0)