separate backends implemented via a 'multi' meta-backend.
Uses config like this:

```
backends: [multi]
multi:
    locked:
        pushover:
            user_key: user-api-key
    unfocused:
        default: {}
    focused: {}

```
This config would cause no notifications if the shell is focused, desktop
notifications when unfocused, and pushover notifications when the screen is
locked. Freaking magic. 🎉

WIP of dschep#125: screen locking detection

* dschep/locked:
  yapf-ify
  WIP of dschep#125: screen locking detection

…reenlock

* commit '84903e7ae7b7ff6ba102558948cd15e5a361a555':
  Add screensaver detection for MacOS

* master:
  virtualenv already installed i think
  use pyenv for py3k
  Switch to pywin32

Fixes dschep#168