Create index.xml for repository

In a private conversation we talked about a GUI frontend to LuaRocks which parses the HTML file from the repository. I noticed some broken HTML (and some missing escaping), so I wrote a module for creating the repository information in XML format and a corresponding XSLT transformation to mimic the current website layout. It does produces valid XHTML 1.0 Transitional, and at least IE 8, Firefox (since forever) and recent Chromes can render it client-side, but there are probably some browsers which don't handle XSLT, so I'm not sure it can replace the current `index.html` file (it is some 40 kbytes smaller, though). @stevedonovan seems to prefer to parse/modify the broken HTML code for his WebRocks project, and I'm not sure I'll work on a GUI frontend in the near future, but here is the code nonetheless ...

An example XML file is available [here][1] (without the actual rockspecs, manifests, etc., so don't click on any links).

  [1]: http://lua.siffiejoe.dtdns.net/rockspecs/index.xml