Add support for tvOS

[nuudles]

I needed a UIStackView in some shared code between my tvOS app and an iOS app, so I've gone ahead and added tvOS podspec support for TZStackView and it seems to work great.

[iGranDav]

@tomvanzummeren is there any reason for not merging this PR?

[tomvanzummeren]

Not really, other than I have no time right now to process the whole list of merge requests for TZStackView and release a new cocoapods version etc.

[iGranDav]

Ok do not hesitate to provide contributor rights if you need help to maintain and test this great library.

[tomvanzummeren]

Hey @iGranDav , would you really like to help me maintain this library? I can give you contributor rights if you're still interested!

[iGranDav]

Hello @tomvanzummeren I would be happy to test and merge some PR but I have strongly limited ressources to test under iOS 7

[tomvanzummeren]

@iGranDav well, thanks to one of the pull requests I merged in the past, the library is no longer compatible with iOS 7 anyway. So you would not be making things worse.

But isn't iOS 7 available anymore as a Simulator?

[iGranDav]

@tomvanzummeren not really supported officially and you need to have Xcode 6.4. But in any case it doesn't work that way because you will be then building under SDK 8. And we want to test on iOS 7 with a SDK 9 build.

[tomvanzummeren]

@iGranDav ok I agree, so it won't be a 100% covering test if you test with SDK 8. But it will be a lot better than NOT doing any testing, right?

Best case scenario: you end up restoring iOS 7 support
Worst case scenario: everything stays the same (no support for iOS 7)

Are you up for it?

[iGranDav]

Let's do this 👍

[tomvanzummeren]

I made you a collaborator!

[iGranDav]

Thanks! I'll be merging this tested one first :)

[tomvanzummeren]

aight! good luck!