Expose answers as part of Api::State

Add State and Previous Question spec
Answers are Model::Answer objects
Fix bug in PreviousQuestionPage generation (incorrect responses were being provided)
Move build_question_pages to private method inside Api::State
Expose answer object instead of response string in PreviousQuestion
Implement #humanize for Salary answer
Implement #humanize for MultipleChoice answer

Fetch the human readable version of each choice, not computer-readable one
Default #humanize to value for answers who don't have it specified
Use regex matching for salary humanization
Merge pull request #73 from alphagov/humanized-answers

Humanized answers