changing pkg function to use new packages.octave.org subdomain

[John W. Eaton]

I checked in the following changes so that the pkg function now looks to 
packages.octave.org instead of octave.sourceforge.net.  For now, we are 
rewriting the URLs to point to source forge, but in the future we may do 
something else.  By using packages.octave.org and rewrite rules now, we 
should be able to make a transition more easily in the future, without 
having to change pkg.m.

http://hg.savannah.gnu.org/hgweb/octave/rev/7ad3eea8a3af

I made the change on stable so that it will appear in the upcoming 3.6.4 
release and we might be able to make a change in the location of Octave 
Forge packages sooner than we would if the change only goes on the 
default branch.  I don't think this is a particularly risky change, but 
if anyone strongly disagrees, let me know.  I've also merged the changes 
to default.