Previously, ActiveSupport::Autoload was global and reserved
for usage inside Rails. This pull request makes it local,
fixes its test (they were not being run because its file
was named wrongly) and make it part of Rails public API.

  * A new module (ActiveSupport::Autoload) is provide that extends
    autoloading with new behavior.
  * All autoloads in modules that have extended ActiveSupport::Autoload
    will be eagerly required in threadsafe environments
  * Autoloads can optionally leave off the path if the path is the same
    as full_constant_name.underscore
  * It is possible to specify that a group of autoloads live under an
    additional path. For instance, all of ActionDispatch's middlewares
    are ActionDispatch::MiddlewareName, but they live under 
    "action_dispatch/middlewares/middleware_name"
  * It is possible to specify that a group of autoloads are all found
    at the same path. For instance, a number of exceptions might all
    be declared there.
  * One consequence of this is that testing-related constants are not
    autoloaded. To get the testing helpers for a given component,
    require "component_name/test_case". For instance, "action_controller/test_case".
  * test_help.rb, which is automatically required by a Rails application's
    test helper, requires the test_case.rb for all active components, so
    this change will not be disruptive in existing or new applications.