Added new primitive - maildir_format().

  This has a default setting of "$CHECK - $PATH", which matches
 what we currently have hard-wired.

  The setting isn't actually used yet, that will come next.

  This is part of issue #41.
Added unistd.h to message.cc because of unlink error
Merge pull request #42 from olafura/master

Added unistd.h to message.cc because of unlink error
  maildir_format() is now honoured.

  The display of the maildir-mode uses the maildir_format string,
 such as:

    $CHECK - $UNREAD/$TOTAL - $PATH

  This updates #41.
  Document maildir_format, and set default value.

  This fixes #41.
  Updated comments around the header-display.

  This is part of issue #43.
  An unknown format-string will be assumed to be a header-value.

  This allows CMessage::format( "$X-Remote-IP" ) , etc.

  Fixes #43
  Updated comments for the on_idle example function.