Padded Autoversions, bug fixes

New Things

• Fixes for boto3 DDB conditional puts
• Update to how the -a (auto-version flag) works. Because versions in credstash are stored in DynamoDB as strings, unpadded int versions were causing sort errors when using the auto-version flag. Now versions generated with the -a flag are left-padded with lots of 0's. If you use your own version scheme, you are not affected. If you use the autogenerated versions, you will want to run the included credstash-migrate-autoversion.py script, which will pad out your existing version numbers, and let them work seamlessly with the new scheme