Add way to specify 'current' path

It used to be always 'current', but you may want to name it something
else, or even leave it blank, so your app structure would be:

  /20140703171632
  /20140703171633
  /20140703171634
  ...
  index.html
  ...other 'current' files

Also, fix working when app_path is empty (in case you want to host your
static web site directly from S3, like http://your_app.s3.amazonaws.com)