Introduce inline Application model

This PR adresses the issue https://github.com/terrestris/shogun/issues/50 and one of the "next steps" agreed on in the PSC meeting on [2014-06-11] (https://github.com/terrestris/shogun/wiki/PSC-Meeting-Notes---2014-06-11#ad-31-next-steps-shogun).

The PR extends the method deriving the App-Context object by adding an ``app`` object. The app object is created as a combination of the logged in user's permissions for MapLayers, Modules and MapConfig and the according permissions of the user's groups. Means in detail the common subset of these objects is returned as app-object.
All this could be seen as an intermediate step towards an persistant application model.

TIA for any review.
We talked about this when we agreed that this `@PreAuthorize` doesn't make sense, right?