[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.
hmm.. the `GetDataDir()` (which must be part of the default backup path) returns a `boost::filesystem::path` instance which is not compatible with `GetArg`'s default argument. Converting the GetDataDir into a std::string first would probably end up in the same "mess".
Or do you see a other solution?