bug 937155 - ability to run ftpscraper in dry-run mode

This is a stepping-stone solution. With this in place, you can now run it like this:
```
python socorro/cron/jobs/ftpscraper.py --date=2013-11-07 --products=b2g
```
Instead of inserting builds it just prints what it would have inserted otherwise. 
Hopefully with this in hand we can, on our local dev laptops, better hunt what the heck is going on sometimes in ftpscraper.