Survey reset

Reset Survey:
=============
Affected files: 
1) config/routes
     Add: 
   <code> post "surveys/:id/reset" => "surveys#reset", as: :survey_stats_reset</code>

2) app/controllers/surveys_controller
  Add Action:
 <code> 
def reset
</code>
    <code>SurveySubmission.where(survey_id: params[:id]).destroy_all</code>
    <code>respond_to do |format| </code> 
     <code>   format.html { redirect_to(course_survey_stats_path(@course, @survey), notice: 'Survey has been reset successfully') }  </code>
    <code>end</code>
 <code> end</code>


3) app/views/surveys/stats.html.erb
  Add Reset buttton:
<code>
    <div class="col-sm-offset-10 col-sm-2" >
<%= button_to "Reset Survey", {action: 'reset'}, class:'btn-danger', data:{ confirm: 'All responded answers will be removed. Are you sure to continue?', disable_with: 'Loading..' }  %>
</div>
</code>

Feature: Lecturer is able to reset his/her survey. Once he reset all the answers will be destroyed. Users are required to do survey again. 

Fixed : https://github.com/Coursemology/coursemology.org/issues/140
Use two spaces for indentation. :smile: 
https://github.com/bbatsov/ruby-style-guide#spaces-indentation