Register Lotus applications

## Registry

When `Lotus::Application` is inherited, register the subclass.
The set of the subclasses are available at `Lotus::Application.applications` (private API).

## Preload

This PR introduces `Lotus::Application.preload!`. This API triggers `.load!` an all the registered applications.

The reason is simple. In full stack applications if we want to test components such as actions or views in isolation, because they reference `Bookshelf::Action` or `Bookshelf::View` and those modules are created at the runtime, we need to trigger the mechanism that preloads the frameworks (`Bookshelf::Application.load!`).

In the _container_ architecture, we have multiple applications, so from `test_helper.rb` we preload all the frameworks in bulk.

Close #30