add external targets to main build script

create/copy external JS and HTML target files:
tiddlywiki_externaljs.html
tiddlywiki_externaljs_tiddlyspace.html
twcore.js
jquery.js
jquery.twstylesheet.js
fixup copy for TiddlyWiki.html beta target

copy index.html to TiddlyWiki.html (for use by TiddlySpace???)