Add LGPL dual license.

I've run in to a situation where I'd like to use Horde3D in a project, but another dependency is licensed under the GPL. The EPL which Horde3d is licensed under is incompatible with the GPL which prevents me from using Horde3D in my project. I brought this up with Volker, citing this old topic in the forums: http://www.horde3d.org/forums/viewtopic.php?f=1&t=744&hilit=gpl&start=90, asking if Horde3D could see a GPL compatible license. He suggested I create an LGPL dual license patch.

I used LogBack (http://logback.qos.ch/license.html) as a reference for the language used to communicate the dual license.

The website page: http://www.horde3d.org/license.html would also need to be updated to reflect this change.