`opam doc -n <pkgs>` runs without a server now (instead of `--no-server`)

This switches to using getopts, which requires bash but works properly
(previously `opam doc pkg --no-server` wouldn't work).
`opam doc-serve` will default to the OPAM root now
Fix opam-doc-serve typo
Merge pull request #91 from avsm/master

Doc script fixes