Prevent KeyErrors on pages without canonical urls

I am getting a KeyError when a page doesn't have a canonical URL tag, on line 77 here:
![image](https://cloud.githubusercontent.com/assets/1697151/3417277/3e75fe46-fe37-11e3-8b03-fe9569a954cc.png)
I think this PR catches any such errors...handle it however you like.
Thanks!