Add respond with empty image

As discussed at RubyConf
Closes #86 
All credit should go for @parolkar I just implemented his changes.
Using constant value is better than string literal.

```rb
EMPTY_GIF_IMAGE = "GIF89a\u0001\u0000\u0001\u0000\x80\xFF\u0000\xFF\xFF\xFF\u0000\u0000\u0000,\u0000\u0000\u0000\u0000\u0001\u0000\u0001\u0000\u0000\u0002\u0002D\u0001\u0000;"

def configure(conf)
  # ...

def on_request(path_info, params)
  # ...
    if @respond_with_empty_img
      return ["200 OK", {'Content-type'=>'image/gif'}, EMPTY_GIF_IMAGE] 
```

We can use this value in test.