Collapsible notebook code cells in liquid_tags plugin

This PR adds the capability to collapse/expand IPython Notebook cells in the html output generated using the liquid_tags plugin by @jakevdp. An example is [in this blog post](http://joergdietrich.github.io/notebook-collapse-input.html). This PR also removes the unused code identified in issue #140.

I'm getting an "Error checking merge status" now. Not sure why. It looks clean to me right now. I'll submit and then we can see what's going on.