Add support for MacOSX storyboards

[phimage]

I have added the support of MacOSX storyboards by

• adding an enum 'OS', which provide strings for output
• reading OS in storyboard files
• moving the code at the end of files in 'processStoryboards' function

I make some test on my two different MacOSX Storyboards (but maybe I don't use all features) and check that there is no output differences after my modification on NatalieExample

[krzyzanowskim]

uderstand. Thank you. Very good work! I was completly unaware of OSX in that regard.

Ill merge this one, and do some refactoring later. I think some parts can be simplified for incrqsed readability for this new dual state.

Wondering how long we'll be able to keep both of these under the same umbrella though :-)

[phimage]

Yes some OS variables like storyboardTypeUnwrap are not clear Maybe sometimes instead of delegate to os:OS (os.XXX), some 'if' or 'switch' could be used directly if os == OS.iOS { println(something) } else if os == OS.MacOS { println(something else) }