Add the Scorm2004::Sequencing::ProcessFactory class.

This class is designed for being used by AbstractProces.
Add the Scorm2004::Sequencing::ProcessFactory::AVAILABLE_PROCESSES constant.
Add the Scorm2004::Sequencng::AbstractProcess class.
Remove some require lines.
Lazily-initialize process_factory.
Add some require lines.
Add Termination Request Process to ProcessFactory::ACTIVITY_TREE_PROCESSES.
Privatize the ProcessFactory#classify method.