gitweb: (gr)avatar support

Introduce avatar support: the feature adds the appropriate img tag next
to author and committer in commit(diff), history, shortlog, log and tag
views. Multiple avatar providers are possible, but only gravatar is
implemented at the moment.

Gravatar support depends on Digest::MD5, which is a core package since
Perl 5.8. If gravatars are activated but Digest::MD5 cannot be found,
the feature will be automatically disabled.

No avatar provider is selected by default, except in the t9500 test.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>