[WIP] Indexing

This is the beginning of notebook indexing in support of search (#252).

Currently I'm making an opinionated view on *how* we learn about an elasticsearch node, configured here through Docker provided environment variables (`NBSEARCH_PORT`). The linked elasticsearch node then needs to be named `nbsearch` to be picked up. In concert with the memcaching layer then, our `docker run` line looks like:

```
docker run --name nbviewer.1.service -p 8080:8080 \
           --link nbcache:nbcache \
           --link nbsearch:nbsearch \
           ipython/nbviewer
```
I don't think you want to hardcode this request