[backups] Automatic backups basics

- allow to do automatic backups when keypool gets refilled
- by default backups are turned off
- by default only encrypted wallets are allowed for backup (configurable)

This could be the basic for later adding backup possibilities for Bitcoin-Qt including visual elements for latest and amount of backups as also validity-check-display of backup files.
You were missing a few months here, so this will be my only nit for you ;). Can you please try to use alphabetical ordering for such includes (see e.g. most other files). This belongs to the top of the boost includes.