Fix group field validation

~~I'm stuck with group fields, and I'm currently thinking of getting rid of them.~~

My first idea was just to allow grouping of fields, in a way that does not affect their individual validation at all.

For example, the following model : 

```javascript
{name: 'person',
 fields: [
    {name: 'age', type: 'int'},
    {description: 'address',
     type: 'group',
     fields: [{name: 'street', type: 'string'},
              {name: 'city', type: 'string'}]
    }
 ]
}
```
could validate this record
```javascript
{age: 12, street: 'bakery', city: 'london'}
```

I must admit that the group field type I had in mind belongs more to the presentation layer (grouping widgets in forms) than the model layer.

The current implementation is completely wrong : it does not validate at all ! (see #189, #192)

I tried to hack around, ~~but I cannot find~~ I found a way to flatten the fields list (i.e. expending the fields at the model level with the ones from the group fields).

~~And if we validate in a dict (e.g. ``{age: 12, '': {street: 'bakery', city: 'london'}}``), then there is no difference with the ``object`` field, which is well tested and rather intuitive...~~

~~Any idea ? Opposition to ``group`` field type removal ?~~