Issue #2351729: Add mockery library to ease testing.
Issue #2351729: Use PSR-0 / PSR-4 namespace loading.
Issue #2351729: Add a service provider plugin manager.
Issue #2351729: Add a ContainerInterface.
Issue #2351729: Add a ContainerBuilderInterface.
Issue #2351729: Add a ContainerBuilder class.
Issue #2351729: Add a stub for a DI container.
Issue #2351729: Add tests for ContainerBuilder class.