Small FAQ additions

Ref #1690
Remove package directories without warnings when they only contain empty dirs

Closes #1740 (not exactly what was asked for, but this seems alright)
Merge pull request #1741 from OCamlPro/add-faq

Small FAQ additions
Merge pull request #1742 from OCamlPro/rm-empty

Remove package directories without warnings when they only contain empty dirs
Include 'repo' in index.tar.gz, and better repo init
Fix repo redirects at init
Merge pull request #1744 from OCamlPro/fix-repo-redir

Makes repo redirects work at 'opam init' and 'opam add'
Add a variable to control over-parenthisation of exprs

for 1.1 compat
1.1 compat mode: ensure all parens needed by the 1.1 parser are present