allowing the 'ws-server' to process plain http GET requests

This is a port of the changes proposed in https://github.com/aphyr/riemann/pull/422

I've switched the uri's to /index and the 'q' parameter is now 'query' to be consistent with the existing websockets uris.

provides:
    GET /index
    GET /index/{host}
    GET /index/{host}/{service}

It's not 100% consistent yet, there is some work I need todo to retrofit some of the new helpers into the websocket handlers.

Hosts and services can be `nil`, which is distinct from `"nil"`. It's not clear how these paths interact with nil hosts or services. I suggest removing both these handlers in favor of the query handler, which provides distinct encodings for both.