lml#341: Fix multiple choice styles.

- Ensure that multiple choices are laid out in columnar fashion.
- Add mark_correct variable to toggle correct answer tagging
  in the HTML element with a class.  This is needed to make sure
  correct answer is not revealed through view source :)

app/views/exercises/_choices.html.erb

@@ -6,19 +6,21 @@
 Params:
 
   exercise: required
+  mark_correct: optional. Marks the correct answer with a class. default: false.
 %>
 
 <%
    exercise_data = exercise.content
-   solution = exercise.correct_choice_index
+   solution      = exercise.correct_choice_index
+   mark_correct  = false if local_assigns[:mark_correct].nil?
 %>
 
 <div class:="question multiple-choice">
   <ul class="unstyled">
     <% exercise_data["simple_question"]["answer_choices"].each_with_index do |answer_choice, ii| %>
-    <li <% if solution == ii %>class="correct-answer"<% end %> >
-       <span class="choice-letter"><%= choice_letter(ii) %>)</span>
-       <span class="answer-choice"><%= answer_choice["html"].html_safe %></span>
+    <li class="row <% if mark_correct && solution == ii %>correct-answer<% end %>">
+       <div class="choice-letter span1"><%= choice_letter(ii) %>)</div>
+       <div class="answer-choice span6 text-left"><%= answer_choice["html"].html_safe %></div>
     <li>
     <% end %>
   </ul>