Support for ~/.aws/credentials

Initial support for fetching the aws_access_key_id and aws_secret_access_key from `~/.aws/credentials` instead of `~/.aws/config`.

This will merge both ini files since currently we're storing the region in the `@credentials` hash as well.

Should solve #41.
Can you please turn this into an if else instead?  unless / else can be confusing to people :)