Update implementation of license

It's the same license, but I've updated the implementation, both to make more sense for free software and better follow the guidelines set out by the license itself.

Based on this how-to: https://www.gnu.org/licenses/gpl-howto.html

I do technically still have to add the contents of the `LICENSE` file to the top of every file in the source code... THAT SOUNDS FUN :^c