License upgrade to GPL version 3 or later

License upgrade to GPL version 3 or later

Why upgrade?  I'm not a lawyer, but here's my take on the differences
to version 2:

* Software patents: better protection against abuse of patents to
  prevent users from exercising the rights under the GPL.  I doubt
  we'll get hit with a patent suit, but it's a good move just on
  general principles.

* License compatibility: compatible with more free licenses, i.e. can
  "steal" more free software for use in Empire.  I don't expect to steal
  much, but it's nice to have the option.

* Definition of "source code": modernization of some details for today's
  networked world, to make it easier to distribute the software.  Not
  really relevant to us now, as we normally distribute full source code.

* Tivoization: this is about putting GPL-licensed software in hardware,
  then make the hardware refuse to run modified software.  "Neat" trick
  to effectively deny its users their rights under the GPL.  Abuse was
  "pioneered" by TiVo (popular digital video recorders).  GPLv3 forbids
  it.  Unlikely to become a problem for us.

* Internationalization: more careful wording, to harden the license
  outside the US.  The lawyers tell us it better be done that way.

* License violations: friendlier way to deal with license violations.
  This has come out of past experience enforcing the GPL.

* Additional permissions: Probably not relevant to us.

Also include myself in the list of principal authors.