Visualizer

Closes #131

@rcrichton if you could please review?

There are obviously still a lot that could be done (including tests!), as well as several TODO items. However as discussed these can be taken on in our next metrics dev iteration.

Note the following on logging: The visualizer thread in the console makes requests multiple times per second, and the log was filling up rapidly:
```
info: No user exists for undefined, denying access to API, request originated from localhost:8080
info: User root@openhim.org made an authenticated requets to the API from localhost:8080
```

I removed logging for the second message and created an issue for the first: https://github.com/jembi/openhim-console/issues/110
I wonder if we should make this its own middleware that comes after the messageStore. This is what I have suggested for metrics in one of the metrics issues. What do you think?