Support for IAM instance profiles

[ajsharp]

This may a limitation of the underlying AWS client, not sure. It would be nice to take advantage of IAM instance profiles instance of needing to provide access keys. I know there's a blurb about this in the README, but it's not clear how to configure asset_sync to take advantage of this if you're deploying to AWS infrastructure that has access to various resources through IAM instance profiles.

[alvinypyim]

@ajsharp, try this:

AssetSync.configure do |config|
  # ...
  config.aws_iam_roles = true
  # ...
end