Moved backups into backupdir/files/<hash>

Storing backups by hash instead of original filename prevents
duplication in the event that a file moves. This update is part
of the preparation for recovery code.
Changed date format in backup log.

Altered from <year>-<month>-<day>.<epoch> to
<year>-<month>-<day> <hour>:<minute>:<second>
Both are at second-level resolution, but one is more human readable.

Includes addition of commandline backup-recovery argument that raises
an exception that indicates that SALVE does not yet have backup_recovery
support, in case it is invoked with that option before support is added.
Merge branch 'feature/backup-name-by-hash' into dev