Start using async / await

[fasterthanlime]

Turns out it's awesome:

• When I'm updating the Electron app in the background the main window pops up for a second. Squirrel/Squirrel.Windows#446 (comment)
• http://pouchdb.com/2015/03/05/taming-the-async-beast-with-es7.html
• http://bluebirdjs.com/docs/api/promise.coroutine.html
• https://babeljs.io/docs/advanced/transformers/other/bluebird-coroutines/

...and supported in Electron (both renderer & browser side)

[fasterthanlime]

This is the kind of issue that'll probably never quite be complete (It's an ongoing effort), but converting these should be good enough to close the issue:

• tasks/*
• task manager in install-store
• util/ibrew
• 7zip extractor