[Bug 847874] Plot topic activity over time.

I had some fun with Rickshaw. I tried to make it fairly reusable, because I bet we will have more graphs on this page later. I worked on making it update the graph with ajax, but I couldn't figure out a sane way to get Rickshaw to update the graph with the new data, so I rolled it back. (So there is no ajax stuff here).

* Updates elasticutils to add support for histograms.
* Shows volume of questions, broken down by topic.
* Pulls data from ES.
* Configurable date range and bucket size.
* Can reorder and hide topics on the graph.

r?