Adding support for IAM roles and AssumeRole for IAM API

I noticed today that the IAM service does not support IAM roles and credentials from AssumeRole.  At first I thought this was a limitation from AWS, but another developer let me know that this works find in boto.  So I've gone ahead and tossed together a patch, which adds this support.  I've tested it using ~./fog credentials and an IAM role.  The unit tests still run correctly also.