[PR] Add the GPLv2 (or later) license to the theme

We need to be explicit, even more so as we start to include code from other GPL projects. In preparation for our use of the builder, I've already added an area for "incorporates work from...".

I'm learning from another project that adding a license late can be a bad idea. :)

@natejacobson - As you and I are the only contributors to the theme thus far, this is pretty easy. I'm okay that all code contributed to this project by me is converted to the GPLv2 (or later) license. If you're also okay with that, merge away!