Use link for contact field

[duckontheweb]

contacts has email required. I'd go the STAC way here to allow (and require?) a URL to be able to link to a more detailed (contac?) page where you usually can get more details. Requiring e-mail could is not so nice because some people may not like to provide one in an easily machine readable way to avoid spamming etc. Why is organization different from name? Usually it seems a general name property is enough to fullfill both personal and organizational contributions. Also, names (and emails) get quickly outdated with people switching companies etc. Roles from STAC could also be useful (host, creator, licensor, etc). So proposal is name (required), url (maybe required), roles (maybe required) and email (not required).

Originally raised in #25