Added 'postfix' role

- this role is not finished yet by a long shot, I just had to stop
  somewhere and commit what I already coded...;

- postfix is a "common" role, present on all hosts by default. You can
  override that by setting 'postfix' variable to False;

- by default postfix is configured as "null client", ie. no local mail
  delivery and all mail relayed to MX for local domain. You can define
  what postfix capabilities should be enabled using 'postfix' list
  variable, see 'defaults/main.yml' for details;

- postfix role tries very hard to be a good citizen, comform to RFC
  specifications and not be an Open Relay. Of course user can totally
  override that;