Update npm-shrinkwrap.json

Temporarily switched to the elasticsearch branch ready for testing on a
vhost, this will need to be updated once the elasticsearch branch has
been merged on popit-api.
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.