Add assume_role() function

[leeper]

Per discussion at cloudyr/aws.s3#65:

• Add a function assume_role(), which assumes a named role, and replaces environment variables
• Add another function to revert to default credentials, maybe release_role() or resume_role(), by retrieving credentials stored in a package environment and resetting environment variables accordingly
• given this is actually part of the STS API, should update docs to reflect that STS is being incorporated here.