Allow s3_id and s3_secret to be omitted

This enables deploying jekyll-s3 on EC2 instances with IAM roles.
The AWS SDK will automatically fetch temporary credentials.
Bump version to 2.7.0
Add David Michael Barr to the list of contributors
Merge branch 'iam-temporary-credentials'

Conflicts:
	README.md
Add doc on working with EC2 and IAM roles
Add an example of defining custom CF configs
Add example configurations
Update minimal conf example
Fix typo in doc
Highlight page speed feature
Add support for simple 301 redirects through config file
Invalidate redirects in CloudFront
Documentation for simple redirects