Default assets paths don't work on Sinatra::AssetPack. It needs to be relative.

The default assets paths are absolute:
```ruby
@image_assets = "#{app_root}/assets/images"
@js_assets    = "#{app_root}/assets/javascripts"
@css_assets   = "#{app_root}/assets/stylesheets"
```
They are not compatible with Sinatra::AssetPack. The line 252 and line3 of sinatra-assetpack/lib/sinatra/assetpack/options.rb shows:
```ruby
tuples = @served.map { |prefix, local_path|
          path = File.expand_path(File.join(@app.root, local_path))
          ...
```
Take js assets for example. @app.root is "/path/to/padrino/root/app", and local_path is just what we defined of @js_assets, which is "/path/to/padrino/root/app/assets/javascripts'. Pay attention to that join method calling. So path variable will be a mess: "/path/to/padrino/root/app/path/to/padrino/root/app/assets/javascripts". That stops AssetPack from working.