Update verifying-signatures.html with info for Mac OS X
The current version of verifying-signatures.html points Mac OS X users to http://macgpg.sourceforge.net/. However, that site points users to http://www.gpgtools.org/.
It seems like gpgtools is some new fancy all-in-one bundle of GPG tools for Mac OS X. I wonder if installing "GPG2" would be sufficient, but I can't test it. It would be great if someone could look into it and update the verifying-signatures.html page if necessary.