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.
Removed 'on_site' keyword argument from 'notification.send' method call.

Reverted testing purpose code modification that had been accidentally committed :P
Fixed uncommented comments row in gas/models/order.py.

Accidentally committed before :P.
Fixed balance_current value update procedure.
Improved pre-installation file.
Improved db backup script and backup guide format.

Added the possibility to set db backup save target directory.
Db backup script howto.

Guide to explain how to run the Gasista Felice db backup script, and where to find the produced archive containing the backup and its metadata.