Added pod spec

Hi Honghao

Your ZHStackView is awesome! Well done!
You should add it to cocoapods.
I've created a pod spec for you. If you accept this pull request, there are only three steps left to add it to cocoapods:

1. Create a 0.0.1 release (or whatever version number you feel like; but it has to be equal to the version number in podspec)
2. Go to the podspec directory, run `pod lib lint` in Terminal.
3. If the podspec passes validation, run `pod trunk push` to add it to cocoapods. You might have to validate your email address but it should be easy.

Regards,
Yichi