Add new Mountain Lion properties to NSColor

Add two new Mountain Lion properties to NSColor ([CGColor](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/ApplicationKit/Classes/NSColor_Class/Reference/Reference.html#//apple_ref/doc/uid/20000353-SW21) and [underPageBackgroundColor](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/ApplicationKit/Classes/NSColor_Class/Reference/Reference.html#//apple_ref/doc/uid/20000353-SW19))