Supplement conventions.asciidoc for inclusion in the book's preface

This PR converts and supplements conventions.asciidoc into a format suitable for inclusion in the book's preface as  conventions. The aim here is to give editors a standard to move all recipes toward.

Notable inclusions are a sidebar on lein-try, the addition of a Typesetting section (to O'Reilly's standards) and a number of re-arrangements/re-wordings (to make it fit the book-style better.) Expect that we'll merge this in a few days.

/cc @thebusby, @levand, @jcromartie