tracking urls

hi, i've had this `urls` branch laying around for a while and just rebased it on `master`. It includes web tracking links, but I'm a little weary because these urls are not guaranteed to be permanent - they may change at some point in the future...

If you choose not to merge this, I think you should remove `Gemfile.lock` from the repo.

http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/