SystemTrayIconComponent docs error

Hey Jules

I've just noticed that the description of the SystemTrayIconComponent class states that the class only works on Windows and Linux, when I belive it is also useable on OS X. Nothing major, but it did just caused me a good 5 min of confusion when trying to find out if Juce used another class to handle this on Mac!

Thanks - I guess the comment was just out-of-date, I'll tidy it up!