Squashed 'emacs.d/async/' content from commit 242ae73

git-subtree-dir: emacs.d/async
git-subtree-split: 242ae734273162f4ec027618610a099c9a73557b
Merge commit '7b0cd3801398fcb47949f68fd3ae183dda47e37a' as 'emacs.d/async'
Factor out OS X things; add isort.