Question Aliases

Unfortunately we're unable to use 'alias' due to it being a reserved word and the way http://kschiess.github.io/parslet/transform.html works. I have renamed the question_alias into alias in all the 

  module Model
    module Element
      module Question

Structs. You're totally right the currently implementation would have only worked for multiple choice questions due to that naming inconsistency. Well spotted! :moon: