Support for credentials profiles

[kimpepper]

The AWS SDK provides the ability to load credentials from a ~/.aws/credentials file. This provides a more standard approach than the currently recommended method in the README.md

http://docs.aws.amazon.com/AWSSdkDocsRuby/latest//DeveloperGuide/set-up-creds.html

[benesch]

I'm fairly certain that if you don't have AWS_ACCESS_KEY_ID set in your environment but you do have a default profile in ~/.aws/credentials, those credentials will automatically be loaded. Is this not the case? (If not, ping me and I'll dig in.)

Either way we can definitely add a note to the documentation.

[benesch]

And you should be able to choose a profile by setting AWS_PROFILE. I'll leave this open until I can verify this behavior works and add a note to the README. (Any chance you'd be able to submit a documentation update PR?)

[benesch]

Confirmed that this works. I'll update the documentation and error messages.