configuration for serving static media off S3

obviously, for production and staging environments it is now important
that you have your local_settings.py set up with the AWS credentials
and that you have made the appropriate ccnmtl-yourapp-static-staging
and ccnmtl-yourapp-static-prod S3 buckets.
Merge pull request #36 from ccnmtl/s3static

configuration for serving static media off S3