Though XML tools should support unicode, some people will be
using SGML tools, and these do not. Using entities makes the
docbook files more portable.
Also refactored encodeEntities and charToHtmlEntity in
HtmlEntities.hs
git-svn-id: https://pandoc.googlecode.com/svn/trunk@394 788f1e2b-df1e-0410-8736-df70ead52e1b