Refactor FsAdaptor for future MockFileDelegate testability.

This removes all direct file and directory manipulation from
FsAdaptor, moving it into the FileDelegate.  All access to
files and directories from FsAdaptor are now done via the
delegate.

A new NioFileDelegate was added, performing most direct file
and directory access using NIO Paths and Files classes.

WindowsFileDelegate extends NioFileDelegate, adding ACL and
Monitor support.

Code Review: http://codereview.appspot.com/38140043