Allow for IAM Role/Instance Profiles for auth

Rather than having to have the aws secret key/aws access key anywhere on
an instance this patch allows for the use of Instance Profiles and IAM
Roles to access S3.

This patch removes the checks that ensure that you provided AWS_*
credentials, and instead catches the S3 exception when your attempts
fail due to AccessDenied.

Since boto has code to handle Instance Profiles I honestly didn't have
to do much, just let boto do its thing without provided keys.

Here's the blog post about this feature:

http://aws.typepad.com/aws/2012/06/iam-roles-for-ec2-instances-simplified-secure-access-to-aws-service-apis-from-ec2.html
Merge pull request #1 from SimpleFinance/iam-auth

Allow for IAM Role/Instance Profiles for auth