Add ReorderingVector and Question.reordering_vector

This provides what is required by #9, allowing a Question to generate a
ReorderingVector which is correct for the Question's anchored Answers.
Make default rake task include rubocop

This includes rubocop in the default task, and makes it fail on error, and
includes the tests as well as the tested code and Rakefile for formatting checks.
Documentation fix for Question.reorder_answers
Merge pull request #10 from mikestok/issue-9

Issue 9