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 (`NBINDEX_PORT`). The linked elasticsearch node then needs to be named `nbindex` 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 nbindex:nbindex \
           ipython/nbviewer
```