Update s3_generic_iam to use security token

It looks like some time between the moment @RevaxZnarf submitted his PR and now, Amazon changed something that prevents the use of security credentials without the security token. So I updated this module to send the token along with the temporary credentials. The problem is that the change to s3_generic() breaks backwards compatiblity (more info in the commit msg) and I'm not sure how to fix that.