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 equal to the version number in podspec)
2. Go to the directory in Terminal, run 'pod lib lint'
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