Added support for symbolic links to zfile

* This was used by FileMQ and previously implemented in the fmq_file class
  that zfile is based on.

* A symbolic link is a 1-line text file ending in ".ln" and lets you work
  with large files by manipulating small symlink files. It eliminates the
  need for copying, in some use cases.
Merge pull request #256 from hintjens/master

Added support for symbolic links to zfile