Starting with this commit, the correct XSD for a Nifty version is the
"nifty.xsd" file that ships with Nifty and is included in the nifty.jar
as a resource.

The correct namespace is "http://nifty-gui.lessvoid.com/nifty-gui" and
you can now finally use this namespace to correctly validate
control-definition and style-definition XML files :)

Nifty XML:
  <nifty xmlns="http://nifty-gui.lessvoid.com/nifty-gui">

Nifty-Style XML:
  <nifty-styles xmlns="http://nifty-gui.lessvoid.com/nifty-gui">

Nifty-Controls XML
  <nifty-controls xmlns="http://nifty-gui.lessvoid.com/nifty-gui">