Add relations fields

Most of the work here consisted in giving a reference to database from validators to field types checking. 
The rest is quite obvious.

# Behaviour 

Can only reference an ``id`` of the linked model. Posting a record of the linked model directly is not supported.

# Code

Could not find any way to take a shortcut ! (app registry ? events ?)

# Storage

Wonder if we should serialize into somewhere more HATEOAS : 

Instead of : 

``abcdefgh``

store (and serve) this : 

```
{id: 'abcdefgh',
 model: 'simple',
 uri: '/models/simple/data/abcdefgh'}
```

Please comment :)