* Preliminary support for HTML Help (target "htmlhelp". Note: No
  support for *.chm yet, I have to download hhc first for testing.

* Made directory/file layout more consistent:
     * All-in-one HTML for foo.xml goes into foo.html
     * Chunked HTML for foo.xml goes into foo-html directory with root index.html
     * HTML Help for foo.xml goes into foo-htmlhelp directory
  For the rationale behind this, read "Cool URIs don't change" at
  http://www.w3.org/Provider/Style/URI.

* Renamed "no-chunks-html" make target to "html-no-chunks"