Fix up for Ruby 1.9 onwards

* Update ruby-mp3info
* Move requires to where they're needed
* Require path no longer includes . from 1.9 onwards
Sort naturally by filename

This maps the way I'm using it, to serve a pre-existing set of files.
Use Builder for XML template

String interpolation is not a reliable way to generate XML.
Use id3v2 comment if available
Create a Feed class
Move 'shows' assumption to one point

This would allow a single instance to serve multiple feeds from multiple
directories within the public directory.
Update Sinatra and use Thin as server
Serve directories in public as feeds

This will need more work if we want to put other stuff in public, but
for now it's OK.