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 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