deprecation: add fine-grained deprecation on models

[MichalHe]

Add fine grained deprecation, alloowing the developer to specify individual fields of a model that are deprecated. For example:

class A(Model):
    deprecated = fields.Boolean()
    a = fields.Boolean()

@add_deprecation('b.deprecated')
class B(Model):
    b = fields.Model(A)
    c = fields.Int()

Accessing (set or get) b_instance.b.deprecated triggers deprecation warnining.

The add_deprecation can be defined as follows:

def add_deprecation(path):
    def decorator(cls):
        if getattr(cls, '_deprecated_nodes', None) is None:
            cls._deprecated_nodes = []
        cls._deprecated_nodes.append(path.split('.'))
        return cls
    return decorator