Gum show multiple authors if there are any

until pelican rev 8e897ea it is possible to have multiple authors for
a post. This patch implements the visualization of multiple authors for
the gum theme.
gum: fix indentation in index.html

add indentation spaces which were missing due to copy-paste editing
gum: retab index.html

tabs and spaces were mixed in index.html, retabbed to spaces-only with tabstop=4
Merge pull request #236 from makefu/gum_multi_authors

Gum show multiple authors if there are any