Improve contract of copyRecursive

[bluekeyes]

copyRecursive works but does not take options and requires a specific set of conditions for the destination. Ideally this will merge directories by default, failing if any files would be overwritten. It can then take the StandardCopyOption.REPLACE_EXISTING option to overwrite files. We need to make sure that this behavior can work consistently between the file tree implementation, cp, and scp.