Add rel canonical to derived copies

I have added code to base.coffee that checks to see if the json returns a parentId. If so, it is a derived copy and a rel='canonical' tag with the parentId is added to the head of the page. If not, no tag. 
This URL will translate to, for example, `cnx.org/031da8d3-b525-429c-80cf-6c8ed997733a`, which isn't quite a full URL and is missing the `/contents`.  For example, `http://cnx.org/contents/031da8d3-b525-429c-80cf-6c8ed997733a` is the URL for physics.

Something like `"//#{location.hostname}/contents/#{parentId}"` will translate to `//cnx.org/contents/031da8d3-b525-429c-80cf-6c8ed997733a`, which should be valid (`//` is shorthand that basically means use whatever protocol you're currently using).