Redesign nece

[tatterdemalion]

Redesign with an underlying "override" engine. Nece basically overrides object attributes with the corresponding ones in translations field. We can create a generic override engine using the same tech. Then we can use this engine to power nece.

[Natim]

I kind of like that, I did something related to that in the admin patch in #10 (see the def clean(self) method).
This allow me not to show mandatory fields twice but make sure to set a default value for them.