Start theme support

Functions now used for printing parts of the index.html file and the stylesheet. These need to be extended to check for the presence of a BPGALLERY_THEME variable and automagically load the associated theme file from /etc/bpgallery.sh/themes/ or /usr/local/etc/bpgallery.sh/themes/ or ~/.bpgallery.themes/

git-archimport-id: arch@sommitrealweird.co.uk--2005-desktop/bpgallery--mainline--0.9.2--patch-7