VerbType subclasses: rm class methods

Instead of having these options be invoked
polymorphically by calling these messages on these
classes, implement factories that encapsulate this
decision.  The will be introduced in the following
commits.
Add PassiveInfinitiveVactory: calculate present_passive_infinitive
Add ImperativeRuleFactory: factory for calculating imperative base
Merge branch 'refactor-verb-types'