[FIX] Make sure pinentry can always be launched by killing gpg-agent on logout

- shutdown-gpg-agent which is started when the user logs in and sleep
  during the entire time, till a user logs out. When they log out, it kills
  an existing instance of gpg-agent.
- shutdown-gpg-agent plist is symlinked into each user account, since it
  has to reside in ~/Library/LaunchAgents, otherwise it's not informed
  of a logout.
Merge branch 'dev' of https://github.com/GPGTools/MacGPG2 into dev