Native credentials

The first patch allows client code to pass real AWS credentials instead of just access-key and private-key, that enables some more advanced use cases, such us http://aws.typepad.com/aws/2012/11/delegating-api-access-to-aws-services-using-iam-roles.html    but retains full backwards compatibility. This patch also fixes some failing tests, all tests are passing now.

The second patch adds .gitignore lines, in line with leiningen 2 file structure