Show title attribute of span elements as mouse over text.

* shr.el (shr-tag-span): New function.
gnus.texi: Fix missing backslash on \\&.
message.el (message-insert-formatted-citation-line): handle finding first/lastname when more than 2 names appear.
Fix missing space.
message.el (message-insert-formatted-citation-line): handle case I broke - LASTNAME, FIRSTNAME.
gnus-spec.el (gnus-parse-complex-format): Use unicode escape for left double angle quotation mark.
format-spec.el (format-spec): Allow spec chars with nil (Bug#14420)