Moved mile to kilogram conversion code from contoller to model, and
made it more DRY.

 On branch master
 Your branch is up-to-date with 'origin/master'.

 Changes to be committed:
	modified:   app/controllers/exzbikes_controller.rb
	modified:   app/models/exzbike.rb
	modified:   app/views/exzbikes/_form.html.erb