No extended checks was done. Only tested if the check passes without syntax error and without failing with no reason

- Suppression du create / delete et des autres méthodes associées
- Suppression des imports qui sont devenus obsolètes

Behaviour changed for EmComponent and childs create method :
	- Takes care of duplicated names at creation
	- Handle all the checks in EmComponent method
Factorization of EmField.get_class_table() method
Added a @property method class_table_name to EmClass

The EmField version cannot be tested yet (drop column not yet implemented for sqlite in Database.sqlalter)

Suppression du init et passage des classes plutôt que les instances pour les fieldtypes dans le _fields

The dates are set in EmComponent::create() for date_create and in EmComponent::save() for date_update
The values are sets to datetime.datetime.utcnow() so we now that EVERY date in the db are in UTC, no matter django, database or even server configurations.
We will have to take care to convert dates from database to django timezone in dates fieldtypes.

Added a classmethod EmComponent::newUid() that determines the caller's class and return a newly registrated UID