Replaced the AWS.eager_autoload! implementation.

This implementation no longer relies on the AWS.register_autoloads
hook, runs faster and is simpler.
Removed the usage of AWS.register_autoloads, now using vanilla autoload.
Removed the legacy autoloader module.
Updated AWS::Record to use autoload instead of explicit require statements to support eager autoloading.
Merge pull request #209 from aws/autoload

Autoload update