Make the websocket url customizable

I don't know if it's the best solution but this should work for our case (described in https://github.com/skaes/logjam_core/issues/54). We could get it to work using Nginx to terminate the TLS, so we needed to use some other address entirely.

I think this change shouldn't affect the current behavior. By the way, any reason to use different ports in OSX than on Linux? Might simplify things.

What do you think?