Implement natural keys

[sveetch]

Is your feature request related to a problem? Please describe.
There is currently no way to use Natural keys with Lotus object serialization.

It is not a functional issue but it is almost necessary to dump Lotus datas, since its models have many relational fields.

Describe the solution you'd like
This is easy as add get_by_natural_key() to managers and get_by_natural_key() to Model, so Django is instructed how it can deserialize/serialize objects using natural keys. Obviously this is only for models that uniqueness constraints.