Fix latex document generation.

 * don't try copying files from not-existent _static/ directory
 * comment-out latex_logo definition and remove references
   to `.static/logo_hi.gif` both from latex_logo and html_logo
 * work-around a buglet in sphinx.writers.latex.LaTeXTranslator
   whose visit_reference method doesn't recognize `irc://...`
   as a URL reference.
Merge pull request #138 from lmctv/sphinx-latex-fix

Fix latex document generation.