Add Answer With JSON

* add: format.html on ajax:success
* add: (.answer-errors).html(xhr.responseText) & status: :unprocessable_entity
* add: ('.answers ul').append('<li>' + answer.body + '</li>')
* add: ('.answer-errors').append(value + '<br/>')
Merge branch 'json_answer' of https://github.com/tropikan4/qna into json_answer