Update npm-shrinkwrap.json

Switched to using popit-api from the npm registry, this makes it easier
to tell which version is currently installed, and also removes the git
dependency when deploying popit.
Add bin/reindex command for updating elasticsearch

This uses popit-api's reIndex command to re-index each instance into
elasticsearch.
Add elasticsearch to .travis.yml services
Add search API documentation

I've tried to add a few example of how the API can be used, potentially
this might need expanding in the future if it becomes apparent there are
things people are doing often with the API, but this seems like a good
starting point.
Add elasticsearch install information to docs

Document installing elasticsearch on the platforms we support. Update
the vagrant provision.sh script to install elasticsearch when vagrant is
provisioned.