mark the code as GPL v2

Rob Allen from Princeton Instrument (RAllen@princetoninstruments.com) told me
that the code is now GPL v2, and asked me to update the license information:

> It should have the same license as the USB driver, feel free to
> change it to "GPL v2" and add the comments at the top that states
> that it is free software.  I was going to do it by was too lazy to
> make a github account and branch the code.
>
> Rob