Don't allow customer to choose shipping methods reserved for backend

It seems that by default all shipping methods can be selected by a customer even if they are meant to be reserved for backend use only (meaning the display_on property is set to back_end). This patch corrects that. However, I am unsure if the template is the correct place to narrow the collection of shipping methods. 

Also, this change seems quite straightforward and probably not deserving of a feature spec... but let me know if you'd like me to create on anyway.