Mark string in checkout template translatable

Fixes #700

oscar/templates/oscar/checkout/shipping_address.html

@@ -18,7 +18,7 @@
     {% if request.user.is_authenticated %}
         {% if addresses %}
             
-        <h3>An address from your addressbook?</h3>
+        <h3>{% trans "An address from your addressbook?" %}</h3>
       
             <div class="choose-block">
                 <ul class="row-fluid">