Fix space identification in Mavericks per-screen spaces

In Mavericks, each screen (by default) has its own set of spaces. This
means each screen manager must keep track of its own space identifier.

Extend ianyh's NSUserDefaults hack [0] to identify which screen each
visible window belongs to, in addition to which space it belongs to.
Correlate the two to determine which space belongs to which screen.

Fixes #139.

[0]: http://ianyh.com/blog/2013/09/24/identifying-spaces-in-mac-os-x/
Merge pull request #145 from benesch/master

Fix space identification in Mavericks per-screen spaces