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.

Not clear from the names or docstrings how `paged-items` relates to `apply-paging`. Perhaps this could be named `query-events` or similar?