Add possibility to retrieve related objects for read_many and read_one

Example of usage:

* in ``rest.py``:

```
tag_children = { 
    'childrenTags': 'tags'
}
tag = REST(DBSession, Tag, children=tag_children)
```

* in ``models.py``:

```
class Tag(GeoInterface, Base):
    __tablename__ = 'tag'
    __table_args__ = (
        UniqueConstraint('name'),
        {"schema": 'tagging'}
    )
    __acl__ = [
        (Allow, 'admin', ALL_PERMISSIONS),
        (Allow, 'editor', ('view', 'edit', 'new', 'delete')),
        (Allow, Everyone, ('view')),
    ]
    id = Column(types.Integer, primary_key=True)
    name = Column(types.Unicode(200), nullable=False)
    active = Column(types.Boolean, default=True)
    l10n = relationship("TagL10n", backref="tag")
    childrenTags = relationship("Tag",
            secondary=tag_tag,
            primaryjoin=id==tag_tag.c.tag_id1,
            secondaryjoin=id==tag_tag.c.tag_id2,
            order_by=name, backref="parentTags")
```

Will show something like
```
{
    "active": false,
    "tags": [{
        "active": true,
        "name": "Artenschutz",
        "id": 31
    }, {
        "active": false,
        "name": "Auenlandschaft",
        "id": 39
    }, {
        "active": false,
        "name": "Naturschutz",
        "id": 58
    }, {
        "active": false,
        "name": "Pioniervegetation",
        "id": 71
    }],
    "name": "Naturschutz",
    "id": 58
}
```