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

The modules defined in lib/aws/core.rb were not getting eager autoloaded.  This is because the modules were actually defined (not via `autoload`), and so their constants were not crawled.  I've updated the list of default modules for `AWS.eager_autoload!` to include these and added a test that ensures `AWS::Core` is autoloaded.

Fixes #279