First pass at HTMLBook Index autogeneration

I've adapted the docbook-xsl stylesheets for index generation so that they are now compatible with HTMLBook. It is now possible to generate an Index from \<a class="indexterm"\> markers in HTMLBook content. 

A couple to-dos for the next iteration:

* Update the markup from dl-dt-dd elements to be conformant with EPUB 3 indexing spec
* Activate handling for role/type to allow for generation of multiple indexes for the same book