Add IAM role support for credentials.

So I've added provisional support for IAM roles. It defaults to using credentials so it shouldn't clobber anyone's existing installs (I tested mine and it worked but ymmv).
![with_credentials](https://f.cloud.github.com/assets/127297/217288/b40e9338-84e5-11e2-97ed-688c41e4a059.png)
![with_role](https://f.cloud.github.com/assets/127297/217289/b54788b8-84e5-11e2-899e-b92d0e00eff2.png)
![with_role_and_help](https://f.cloud.github.com/assets/127297/217291/b77c8930-84e5-11e2-919c-9016158d8a25.png)