If no credentials are specified, try and retrieve some temporary credentials via STS and the IAM Instance Profile in EC2

[gozer]

If not available, continue attempting to retrieve credentials the same way as
before

Fixes #16

[okigan]

This maybe a bit too big of a dependency to add to awscurl, meaning if you got boto one should use "boto way" to make aws requests. awscurl aims to give simpler surface area for making signed aws requests.

For now I'd defer to aws cli suggestion of using temporary credentials (from sts or not) and just pass them to awscurl
http://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_temp_use-resources.html#using-temp-creds-sdk-cli

export AWS_ACCESS_KEY_ID=AKIAI44QH8DHBEXAMPLE
export AWS_SECRET_ACCESS_KEY=wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY
export AWS_SECURITY_TOKEN=AQoDYXdzEJr...<remainder of security token>

awscurl ...