Show request headers

Show request headers in the console.

```bash
$ curl -H 'X-Requested-With: XMLHttpRequest' -d "message=hello" localhost:3000

"POST http://localhost:3000/ (2013-12-27 14:19:24 -0800)"
{
              :host => "localhost",
                :ip => "127.0.0.1",
              :port => 3000,
    :request_method => "POST",
            :scheme => "http",
               :url => "http://localhost:3000/",
      :query_string => "",
              :body => "message=hello",
    :content_length => "13",
        :media_type => "application/x-www-form-urlencoded",
           :referer => nil,
        :user_agent => "curl/7.30.0",
               :xhr => true,
           :headers => {
                 "HTTP_VERSION" => "HTTP/1.1",
              "HTTP_USER_AGENT" => "curl/7.30.0",
                    "HTTP_HOST" => "localhost:3000",
                  "HTTP_ACCEPT" => "*/*",
        "HTTP_X_REQUESTED_WITH" => "XMLHttpRequest"
    }
}
```