[x] Add a function assume_role(), which assumes a named role, and replaces environment variables
[x] 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
[x] given this is actually part of the STS API, should update docs to reflect that STS is being incorporated here.
Per discussion at https://github.com/cloudyr/aws.s3/issues/65:
assume_role(), which assumes a named role, and replaces environment variablesrelease_role()orresume_role(), by retrieving credentials stored in a package environment and resetting environment variables accordingly