Be consistent with
Update backup script to make better use of mongodump

- Previously we were creating empty backups
- Specify which database to backup (just backdrop for now)
- Write out dump to default dump folder
- `tar gzip` the contents of dump folder and write out in new format that includes db name (e.g. `2014-03-10-backdrop-monday.tar.gz`)
- Clean up afterwards by removing dump dir

*NB:* mongodump documentation is a bit unclear on this **grumble grumble**
Merge pull request #275 from alphagov/mongodump_backdrop

Update backup script to make better use of mongodump