Attempt to keep websocket alive with empty frames

Fixes #225.

@jrconlin, we had originally discussed just sending the tracking command more often, but I realized that won't actually help keep the socket alive if the device isn't responding. So, instead, I'm sending an empty websocket message every 45 seconds. It *seems* like the server is just ignoring them. I'm not sure it'll actually help keep the connection through the load balancer alive, but it seemed like the simplest solution. What do you think? Want to do something more formalized?
These are already pretty self documenting, but bonus points for the friendly "// 45 seconds" comment (and "// 60 seconds" on the previous line) for future travelers.