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

comment selected_fields because it clashes with ._fields[]
 => we have to rethink of ._fields[] and fieldtypes

Model._components now contains only one dict ( 'uids' key ) and the others are now list of EmComponent instances
Model now handle EVERY access to the _components dict

- Suppression des imports obsolètes
- Suppression de la méthode create

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

 + pep8 and pylint

subordinates implementation is note tested

Not tested because of bugs in EmField

Changed comments about EmFieldGroup.fields()

Now _fields and table MUST be class attribute.
Its important to note that _fields now consist in a list of (name, ftype) tuples and that ftype is a EmFieldType class and NOT an EmFieldType instance

- Added a _fields property (also added it as class property, but the way its done is not satisfactory... we want an ordered dict and not a tuple list as class property)
- Its now allowed to give an uid or a name as em_class for EmFieldGroup.create(name, em_class).
- Now when EmFieldGroup.create is called with an existing name it raises an error (ValueError)
- Removed EmFieldGroup.populate and EmFieldGroup.save

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