README: mention that opam2web is published via opam.

Addresses ocamllabs/opamfu#1.
Expose a number of modules in an installed ocamlfind archive

This makes it possible to use some opam2web routines as a library from e.g. ocaml.org scripts
Display OS constraints on package pages

Fixes #83.
Fix non-root favicon bug
Render issue tracker links on package pages

Fixes #36.
Merge pull request #93 from ocaml/1.3

Merge 1.3.1 changes to dev branch