Switch monochrome systray icon color depending on systray brightness

I am working on issue #2215 in this branch.

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 ![black-on-dark](https://cloud.githubusercontent.com/assets/2794645/4776282/55b2777e-5bc0-11e4-8e30-df6062cf05c2.png), thus rendering it nearly impossible to spot.

This patch checks the NSUserDefaults to check whether the user has chosen the black dock and if so, will use the white monochrome versions for the systray icons.
White icon on dark menu bar: ![white-on-dark](https://cloud.githubusercontent.com/assets/2794645/4776258/0ebf38fc-5bc0-11e4-98e0-9292846a8c64.png)

Right now, the systray icon will not change "live", that is it will stay in the old color when the user changes the dock/menu bar brightness during execution of the app until either the icon image changes (because of state change) or the app is restarted.

The patch needs to be tested on Unix and Windows systems, unfortunately I don't have a development machine setup for those operating systems at the moment. The systray icon behaviour should be unchanged for those operating systems and for earlier versions of OS X.