Simple data storage

Praefectus should store its data somehow. We'll need to decide, how to implement the data storage. Points of interest:
- create a storage abstraction (since eventually I want to support different storage kinds for different tasks, e.g. a server instance of Praefectus may use PostgreSQL while a (mobile) client may use SQLite)
- do we need SQL at all? Could we use NoSQL? Are there any reliable NoSQL solutions?
- seek for the best managed storage solution (LiteDB? SQLite.Managed? What else?) since I prefer not to rely on any native code unless absolutely necessary

Depends on:
- [x] #5