Install man-pages on 'make install'

You'll need to 'make man' first though
Cleanup manpage generation

* removed version-controlled manpages (thanks to cmdliner, they are generated without external dependencies)
* removed opam-check manpage (opam-check is not even installed, and it was almost empty)
* added opam-installer and missing opam-admin manpages
* updated .gitignore
opam-installer: automatically select target dir of manpages

based on their extension
Merge pull request #1450 from OCamlPro/man-pages

Better handling of manpages (build of OPAM _and_ .install files handling)