Add dry run, support large buckets

- --dry-run for tableslurp so you can actually kind of test backups
- Previously if your bucket prefix had >1000 keys, you would not restore
  the latest backup using auto-mode
- Add some debugging

Known bugs:
- Include/exclude is applied against full paths for the backup, but only
  the basename for restores, if you're not using --global-index
- With --global-index, you have to restore the entire backup, where
  having the ability to restore a single table should be possible (but
  semi-complicated). Maybe something like --include for tableslurp.
Merge pull request #6 from urbanairship/add-dry-run

Add dry run, support large buckets