AWS.eager_autoload! now autolaods all of AWS::Core.

Fixes #279
Removed the need to specify modules by crawling all constants when eager autoloading.
Reverted the return value from AWS.eager_autoload!
Updated unit tests to not rely on return value from `eager_autoload!`.
Updated the documentation for eager_autoload! to a void return.
Replaced usage of === with == when checking for nil.
Merge pull request #280 from aws/eager-autoload-fix

AWS.eager_autoload! now autolaods all of AWS::Core.