Add opam and META files, add '-dir' to hol.sh, other updates

[aqjune-aws]

This patch

• adds opam and META files.
• add '-dir' to hol.sh: 'hol.sh -dir' prints its HOLLIGHT_DIR variable. This is convenient when one wants to know where hol light is installed :)
• increases version at hol.ml to "3.0.0+" ('+' is to describe that there are updates after the official release)
• updates README to explain how to use OPAM to install HOL Light

The contents of 'opam' file is equivalent to the already published version to the OPAM repository https://github.com/ocaml/opam-repository/blob/master/packages/hol_light/hol_light.3.0.0/opam modulo:

• A reference to the released tar.gz
• extra-source "META" because the META file will now be included in hol-light with this patch
• Version number and some extra stuffs (they seem to extract those from the directory path)

META was necessary to help ocamlfind locate package 'hol_light'. When HOL Light is installed during OPAM installation, I uploaded this file at gist.github.com and download it; but I think it is a wiser idea to explicitly put it in this repo. :)