Added annoying swp files :)
Admin interface for the services app.

To manage:

* creation of periodic services
* creation of periodic services subscription

Needs (file) e-mail backend into the settings to send emails
to the customers which do not pays the fixed fees.
Improved services admin interface

* added ServiceSubscriptionPayement modeladmin
* added form to fill ServiceSubscriptionPayement paid_for field with
  Service default period

cleaned other modeladmin classes
Internationalization and model modification

Added field period_deadline_modifier to set the tolerance of the period payment delay.
Solved bug on the services payement check

Tolerance should be (arithmetically) added to the last_paid_for (the expected deadline),
not to the according elapsed time from the last payement !
Locales for italian language
Migraton to add period_deadline_modifier Service model field

done with manage.py makemigrations, apply with manage.py migrate
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.
Replaced datetime with django timezone to keep dates awareness.

Added property in ServiceSubscription to check if subscription is expired
Fixed bug prone behaviour of ServiceSubscription "expired" property
MIgrations for services
Deleting unuseful migration