dist: substitute ```@VERSION@``` and ```@DATE@``` tokens

During "make dist", iterate through README and all the man pages
substituting ```@VERSION@``` with the current version number and
```@DATE@``` with today's date (in the form YYYY-MM-DD).

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
README: Add ```@VERSION@``` and ```@DATE@``` tokens.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
man: add ```@VERSION@``` and ```@DATE@``` tokens to all man pages

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>