provider/aws: refactor to use the schema.Provider approach

With this refactor the AWS provider is updated to use the schema.Provider approach released with TF 0.2.

Please note that I don't actively use AWS myself, so the correctness of this refactor/update is based on the acceptance tests that are available for this resource.

Note that this PR is dependent on a little PR on the mitchellh/goamz package (https://github.com/mitchellh/goamz/pull/155).