fix OSX keybindings (super+space doesn't work)

`command` + `space` is a global keybinding in OSX to trigger
Spotlight. I chose `shift` instead of `ctrl` or `alt` as the
modifier key because those are popular shortcuts for the Alfred
app (http://support.alfredapp.com/kb:cmd-space)
Merge pull request #372 from defeated/patch-1

fix OSX keybindings (super+space doesn't work)