Fix typo.
Memoize list calls in resource record set updates.
Don't include resource_records if it's empty.

This fixes a bug where attempting to delete an alias record set would
fail because resource records was passed as the empty list.
Removed old and no longer valid documentation about using session tokens with DynamoDB.
Updated bundled ca-bundle.crt file.
Added support for following 307 temporary redirects.

Fixes #133
References #172
Merge pull request #177 from ab/route53-empty-resource-records

Route53 empty resource records