Not requiring `aws` prefix seems to be causing confusion

Although it seems like e1ven is reporting broken commands, I think the following report actually shows some confusion with not having to prefix commands with `aws`. Muscle memory could be an issue for veteran users. Also, I'd suspect you won't always have aws-shell up (maybe you're ssh'd into multiple EC2 machines doing various things which might/might not include aws-cli-backed-commands). The command syntax difference between `aws-cli` and `aws-shell` could lead to confusion in these cases.

> e1ven
>
> Several commands don't seem to return what I'd expect. With the command line, "aws s3api list-buckets" gives me a list of buckets.. With the aws-shell, this gives me "aws: error: argument command: Invalid choice, valid choices are"..

[Reddit Source](https://www.reddit.com/r/sysadmin/comments/3x2wns/awsshell_the_official_supercharged_aws_cli/cy14fyu)

Note: `s3api list-buckets` works for me.

The issue names no files or tests. Start by tracing how aws-shell parses `s3api list-buckets` and decide whether the intended interface requires an `aws` prefix; done means the behavior and compatibility expectations are documented and tested.

Written by the indexing model from the issue text.