First implementation of the PgBackMan shell to interact with
PgBackMan.

- Display, registration and deletion of backup_servers
- Display, registration and deletion of pgsql_nodes
- Commands quit, shell and help.

First implementation og the PgBackMan database schema

- All tables
- Functions for display, registration and deletion of backup_servers
- Functions for display, registration and deletion of pgsql_nodes
- Jobs_queue