Add formatted timestamp to the default backup file name.

Please add a formatted timestamp to the default proposed file name - to increase usability.
Another usability might be to use a "/backup" subdirectory too.

So instead of:

```
 /path/gitbucket-database-backup.zip
```

this might be better

```
/path/backup/gitbucket-2016-06-07_14-35.zip
```
