make sitediff work as gem and in bundle exec, refs #10
allow local files in before and after, refs #10
purge dead code: lib/sitediff/page.rb
remove dependency on python-beautifulsoup, refs #10

  Instead use an XSL transformation,
  see lib/sitediff/util/pretty_print.xsl
more fixups re:unicode, refs #6
update Dockerfile as per changes, Makefile still deprecated, refs #10