Force world-readable permission on files and directory of the
themes installed by pelican-themes.
Only on posix system i.e. mostly non Windows
Rationale:
If the theme's files have only -rw------- permissions, once installed
system wide and owned by root, they will not be accessible to any user but only
root.
I disliked having to do:
$ pelican-theme -r <theme-name> -i <theme-path>
So I modified install() to handle an upgrade of an existing theme. While doing
so, I noticed that in install() and symlink() the script would error with 'no a
directory' instead of 'not a directory'. So I fixed that for you as well.
