Add ability to generate pages in a seperate directory

If PAGESDIRECTORY is set, write all pages except the index in to that directory

To accomodate this, filename in the generate page is automatically set to the right path, for anything else (e.g. linked stylesheets, or the stylesheet generated by bpgallery, use ${BASEURL} infront of the filename and it'll automatically get ../ when required.)

Also adds a (not very good) changelog - will improve that later!

git-archimport-id: arch@sommitrealweird.co.uk--2005-desktop/bpgallery--mainline--1.1--patch-10