Changed "Quantità" to "Costo" as translation of Service amount
Modified the attributes of some Service fields

* period and period_unit_display are not null
* period_deadline_modifier is valued as 0 if blank
Fixed period_unit_source field

* fixed wrong transaltion
* hidden field in Admin ModelForm
Modified ServiceSubscription model and admin

* blanked subscribed_until field for admin
* excluded last_paid_on and last_paid_for from ModelForm in Admin: these fields will be gotten from
  ServiceSubscriptionPayement
ServiceSubscription fields filled at payement

When a new ServiceSubscriptionPayement is saved, its paid_on and paid_for
values are used to overwrite the corresponding ServiceSubscription last_paid_on
and last_paid_for field values
Fixed naive indentation error :P
Fixed ServiceSubscription bug

The last_paid_for and last_paid_on values were not nullable (while
they are not valued on creation but on relative payement creation)
Fixture to add referrers group

Apply with: manage.py loaddata fixtures/groups.json

or install as initial data renaming the fixture as initial_data.json. This behaviour is
deprecated since version 1.7 of Django (due to migration integration)
MOdified the ServiceSubscription last payement check

Added a control to the check: for now it is possible to consider only
periods which have epoch_now as source.