get remote-host's IP Address

I've made a pull-request for issue #103. It includes update for README and specs.

## Usage
```ruby
EM::run do

  EM::WebSocket.start(:host => "0.0.0.0", :port => PORT) do |ws|
    ws.onopen{|handshake, connection|
      puts "connect from #{connection.remote_addr}"  ## => "127.0.0.1"
    end
  end

end
```

I'm wondering if you like this.