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.