Patch to make asset_timestamp respect public_folder setting

Currently asset_timestamp just uses Padrino.root + "public" + appname as the asset folder. Normally this is equal to the public_folder setting, unless someone overrides it in their app.rb. This fixes that issue by combining the source path with public_folder instead.

NOTE: This fixes the asset_folder bug in my previous pull request. I am aware this fails some tests which it shouldn't. Could someone please look into that for me, I am short on time.