Add rudimentary theme support

Should now look for functions named bpgallery_${BPGALLERY_THEME}_{head,tail,thumbsline,stylesheet} and use them for generating the index.html and stylesheet files. If the functions don't exist it defaults to the theme called default, which is contained in the script.

git-archimport-id: arch@sommitrealweird.co.uk--2005-desktop/bpgallery--mainline--0.9.2--patch-8