AWS IAM Instance profile support (squashed)

replaces https://github.com/wal-e/wal-e/pull/80

cc @wal-e/committers

Add's a high-priority argument, --aws-instance-profile, which instructs wal-e to retrieve user credentials from the aws instance metadata service.

We've been running this in production for the past week with no issues, so I believe this is ready for review.