Document some common issues with GPG on Mac OSX

Deals with this issue: https://github.com/technomancy/leiningen/issues/1386
Remove citations and change 'voice'

Solutions via people from this issue: https://github.com/technomancy/leiningen/issues/615
Remove inline code blocks
Merge pull request #1576 from Christopher-Bui/patch-1

Document some common issues with GPG on Mac OSX