Use "Security Token Service" for access (request side)

[aristidb]

Allow making requests using STS (http://docs.amazonwebservices.com/IAM/latest/UsingSTS/Welcome.html / http://docs.amazonwebservices.com/STS/latest/APIReference/Welcome.html).

This issue is NOT about full support for STS, just making requests using these temporary credentials. There are two caveats:

1. These tokens expire, so there needs to be a way to specify refresh methods - before the token has expired.
2. There needs to be an additional request parameter it seems, the "SessionToken" which needs to be passed in addition to the supplied temporary access key ID and secret access key.

This is especially interesting in conjunction with #29 and #37, so temporary credentials can be periodically obtained with the EC2 instance metadata.