Link to ipynb

This adds a box to the top-right of the HTML view, with a link to the ipynb file. I hope to add a 'Download as .py script' link later, but that will be more work.

At present, the box floats 'above' the normal content, but this is just a matter of styling. Here it is overlapping a code cell:

![Image](http://i.imgur.com/5YDZr.png)

I also cleaned up the 500 error page a bit.