Added mongorestore feature. Can now restore with -R option providing filename_on_s3.tar.gz as second argument to script

Hey, 

I needed a feature to restore from S3 backups too. I thought of building into this script. I know you've even explicitly named to be "backup" script, but still if you think restoring is good enough, please merge it. If you think my code stinks and you can't merge, please inform me so I can improve.