IAM role credentials support

[2xmax]

At this moment, aws.go supports explicit Key, Secret pair access only.

If you use IAM role you can get the temporary credentials without explicit configuration. This credential contain extra "token" field.

IAM_ROLE=$(curl -s http://169.254.169.254/latest/meta-data/iam/security-credentials/)
# will return (key, secret, token) triple
curl -s http://169.254.169.254/latest/meta-data/iam/security-credentials/${IAM_ROLE}

By the way, it is exactly the same way how aws s3 cli, s3cmd tool works: it will use your IAM account by default.

Does it makes sense to add IAM roles authentication feature?

# to aws.go:
func IamAuth()
``

[2xmax]

Sorry, found this issue in another fork. Seems it has already been implemented.