FIX alla guida di installazione

* Strutturata in modo più diretto: evitare giri di parole
* Creata sezione FAQ per approfondimenti
* Separato anche il suggerimento del virtualenv
Messi a punto default_settings.py e settings_dist.py

* Separati secondo le politiche beFair (chiariti anche nella guida di
  INSTALL ora)
* Modificati alcuni valori di default
Discount is optional -> default to 0
ModelForm.Meta.exclude works like a charm. Hidden invoice_period parameter.

Invoice period value can be set to 1 for all subscriptions
Clean per period_deadline_modifier
Modificato template email di base
Lista di cose da fare alcune insieme...
Compute topay start and end with timezone.

Added subscribed_from_dt and subscribed_from_value fields
Needs migrations
Added email field to customer. Needs migration
Removed bugged (and not necessary ) LOCALE_PATHS from settings
Date check in bulk-payment view
Improved development documentation
Changed PercentageDecimalField from form to model field. Clean is done into models.
Set descriptive help_text for Service period_deadline_modifier
Typo: remaind instead of reminder
Added "service_details" and "howmany" fields to ServiceSubscritpion
Adedd CurrencyFields on models
Put Meta class on the top
Fixed wrong conversion unit mapping
Hided "is_deleted" of ServiceSubscriptionAdmin for non referrers