Switch monochrome systray icon color depending on systray brightness.

https://github.com/owncloud/mirall/issues/2215

As of version 10.10 Yosemite, Mac OS X offers the user the option to use
a dark version of the dock and menu bar. The systray icon was black even
on the dark menu bar, thus rendering it nearly impossible to spot.
Merge pull request #2393 from svkurowski/issue-2215

Switch monochrome systray icon color depending on systray brightness