Prep for #22

Refactoring validation code to use the chain of responsibility pattern.
The validator will instantiate a series of validation handlers each
responsible for one type of validation.  At the moment only a single
handler is available, to Minimum Password Length.
More Prep for #22

The Needs Number validation has been refactored into a Handler. The
service locator has been updated to chain the Needs Number validator to
the Minimum Length validator. The Validator has removed the specific
needs number validation code (as it is now in the handler) and the
return type has been changed to a bool (as there is no indication which
handler a failure came from, and in retrospect it probably isn't that
important. What is important is that the pasword passed or failed the
validation as a whole)
More prep for #22

Needs Letter is now a handler, so the code in the Validator class can be
removed for that too.