Minor update to installation document
Moved gas_details data from the formset_class to the context.

Moved gas_details data from the formset_class to the context, in order to make the template do the rendering work.
GF DB backup guide.

- wrote bash script to:
    1. create a META file which contains the last GF commit (hash) and current date
    2. dump GF DB basing on settings.py information
    3. put META and dump files into a tar.gz which is rotated to keep an history of backups
- wrote a backup guide which explains how to restore GF DB to a previous version, assuming to have the tar.gz file introduced above.
Merge branch 'master' of github.com:feroda/gasistafelice
Install guide

Rearranged install guide. Now there are three differetn guides:

* prerequisites (needed by both the following guides);
* user install;
* developer install.
GF backup guide.

Step by step howto to restore GF database linked to a particular commit.
The restore can be done trought a script file placed in <GF_DIRECTORY>/extra/backup_db.sh.
Disabled notifications (For the moment).

To restore when django_notification will be updated.
Merge branch 'master' of github.com:feroda/gasistafelice
Updated UPGRADE procedure to 0.9.9.9
Fixed "got an unexpected keyword argument 'on_site'" error on notify_order_state_update signal.

Removed 'on_site' keyword argument from the notify_order_state_update signal notification send. Added 'on_site' to the signal 'extra_content' instead.
Added exception handling to intergars order form.

An exception is raised when someone tries to create an intergas order with one or more GAS which do not have active pacts with the same suppliers.