Update GPL headers with link to GNU Licenses webpage

Issue #129 complains that the FSF's address in the GPL headers is wrong.  The changes here fix that by linking only to the GNU Licenses page, and not mentioning any physical addresses.

This new headers are practically identical to [the ones recommended][gpl3:howto] by the GPL3 page.

[gpl3:howto]: https://www.gnu.org/licenses/gpl.html#howto