Extend license from GPL-2 to GPL-2 | GPL-3.

Gabor has already agreed, still have to wait for Jeff though. (Assuming
that he is ok given that xts uses that combination of licenses as well.)



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/zoo@977 edb9625f-4e0d-4859-8d74-9fd3b1da38cb