Extends the public user api

While reviewing owncloud/search_lucene#1 I noticed that the public (object oriented) api for user management is lacking severly.

- Adds public interfaces for User objects and the UserManager
- Extends `IUserSession` with `getUser()` and `setUser()`
- Make the user manager available in the public server container

cc @DeepDiver1975 @PVince81