[boost] [proto] reorganized users' guide

I've just completed a major reorg of Proto's docs, with significant portions clarified or completely rewritten. I intend to merge these changes to the release branch soon. I encourage any and all interested parties to click around and offer feedback. The purpose of the reorg was to make the users' guide more approachable and less reference-y. It takes the view that a library built with Proto is a compiler for a DSEL, and as such it has 3 parts: front-end, intermediate form, and back-end. The guide is organized accordingly. You can find the new guide here: http://boost-sandbox.sourceforge.net/libs/proto/doc/html/index.html TIA, -- Eric Niebler BoostPro Computing http://www.boostpro.com