Foreign key definitions

*Title: Foreign key definitions

*Incentive: 

Make it possible to create automated data integrity checks,
and automatic data migration. By knowing the foreign key relations,
automatic key translation will be possible across installations.


*Documentation:

For PHP classes extending eZPersistentObject, they must add to the definition()
function, a few lines describing which fields are foreign keys.
- foreign_class, foreign class name
- foreign_attribute, foreign class key/field name
- multiplicity, describes the number of relations with the foreign class/key

*Example:

In function definition():
...
'foreign_class' => 'eZOrder',
'foreign_attribute' => 'id',
'multiplicity' => '1..*'