Add bin/setup step for installing gnupg and gpgme on OS X

Probably people running this are super savvy and have gnupg and gpgme already installed.

But, if you didn't, the `bundle install` with appear to succeed, but you will [encounter a somewhat mysterious `innappropriate ioctl for device` message](http://stackoverflow.com/questions/21172576/ruby-gpgmeerror-inappropriate-ioctl-for-device).