use npm to get wiki-client for client side javascript

This parallels a similar modification and pull request to the wiki repo: https://github.com/WardCunningham/wiki/pull/23. We now have federated wiki spread over three repos:

* Smallest-Federated-Wiki for ruby version and openId login.
* wiki for node version and persona login.
* wiki-client for client-side javascript and mocha tests -- https://github.com/WardCunningham/wiki-client

Building a client.js composite file remain the responsibility of each server repo.

We expect to see more separation of repos with separation of most plugins a likely next candidate.