Add the Scorm2004::Sequencing::SequencingRuleCondition class.

Implemented its description. Behavior implementation is work in
progress.
Add the Scorm2004::Sequencing::SequencingRule class.

Implemented the description.
Validate Sequencing Rule Condition using predefined vocabulary set.
Add the Scorm2004::Sequencing::SequencingRuleCondition::SatisfiedEvaluator class.
Fix the bug that SequencingRuleCondition::SatisfiedEvaluator may return nil.
Introduce the two new shared_examples to sequencing_rule_condition_spec.rb.

* 'activity with referenced objective'
* 'rule condition'
Add the Scorm2004::Sequencing::SequencingRuleCondition::ObjectiveStatusKnownEvaluator class.
Add the Scorm2004::Sequencing::SequencingRuleCondition::ObjectiveMeasureKnownEvaluator class.
Extend SequencingRuleConditionEvaluator using Forwardable.
Add the Scorm2004::Sequencing::SequencingRuleCondition::ObjectiveMeasureGreaterThanEvaluator class.
Add the Scorm2004::Sequencing::SequencingRuleCondition::ObjectiveMeasureLessThanEvaluator class.
Alias the activity attribute in the SequencingRuleConditionEvaluator class.
Add the Scorm2004::Sequencing::SequencingRuleCondition::CompletedEvaluator class.
Rename duplicated shared examples in sequencing_rule_condition_spec.rb.
Add the Scorm2004::Sequencing::SequencingRuleCondition::ActivityProgressKnownEvaluator class.
Add the Scorm2004::Sequencing::SequencingRuleCondition::AttemptedEvaluator class.
Add the Scorm2004::Sequencing::SequencingRuleCondition::AttemptLimitExceededEvaluator class.
Fix typos.
Add the Scorm2004::Sequencing::SequencingRuleCondition::AlwaysEvaluator class.
Fix the spec bug referencing the wrong shared examples.