recv mode now supports a different layout: filesystem

This layout creates the zookeeper tree into the filesystem. The only difference is that files are named .data. Example:

  zkcli $ create /foo null
  zkcli $ create /foo/bar contents
  $ tractorbeam --layout filesystem --path /foo --output /tmp/zk
  $ find /tmp/zk
  /tmp/zk
  /tmp/zk/foo
  /tmp/zk/foo.data
  /tmp/zk/foo/bar
  /tmp/zk/foo/bar.data