nsqadmin: refactoring

`nsqadmin` is a Go web app, which has some tradeoffs. It's great because it's dead simple to deploy. It's terrible because it's a PITA writing Go based web apps.

I think an ideal middle ground is:

1. `nsqadmin` becomes a JSON API with convenience endpoints to query the cluster and aggregate data
2. `nsqadmin` UI is a client-side app
3. all the static assets for (2) are bundled into the binary so it's still easy to deploy (this is already the case, just stating the obvious)

Simultaneously, it could use a facelift, but this refactoring alone makes things much easier to extend and for others to contribute.

cc @visionmedia