force list context

This permits such code as:

    is($authors->id, 2, 'two authors found')

Otherwise, scalar context is applied to the method, and sort() doesn't work
properly in scalar context.
warn when Gravatar::URL is not installed; add it to recommended prereq list
new author lists added to the list
fall back to <id>@cpan.org for gravatar

This is the same code as used by metacpan in MetaCPAN::Web::View::HTML
at https://github.com/CPAN-API/metacpan-web
Merge remote-tracking branch 'ether/topic/author_dists'
Merge remote-tracking branch 'ether/topic/default_gravatar'
Merge remote-tracking branch 'ether/topic/gravatar_not_installed'
Merge remote-tracking branch 'ether/topic/id_list_context'