Better checks for datasources initialisation
The dyncode were not bootstraped by bootstrap.site_load() function

Now it seems to work. Tested using :
dummy datasource, no handled sites, ipython ui

As a primary_key field has to be unique, added a check at DataHandler
instanciation to make sure a field set as a pk is not set as non-unique

We delete all results with the same UID...

get_referenced method were implemented for SingleReference and MultipleReference class
They are able to return LeObject instances refereced by them

Hook names were wrong

Deleted old methods that are unused (one of them was triggerring an error will doing nothing)

**kwargs was not given to super() constructor

- a "normal" import was left in lefactory_common
- the hook triggering datasources init in dyncode was not called
- dyncode was not exposed in the loader (making hook call useless : the hook has to be called once the dyncode has been "seen" (imported) by python)

Test passes