More lenient parser

Useful for forward compatibility
Add a 'bug-reports' field. Useful for OCamlot triaging.
Doc, License, Homepage and Maintainer fields can now take a list of strings
Be a bit less lenient in the pretty-printer

we really want to distinguish between:

    build: [make "install"]

and

    build: [["./configure"] [make]]
Merge branch 'master' of github.com:OCamlPro/opam