TZStackViewAlignment compatibility and better iOS 7 support

[andi357]

This PR is a fork of PR #60 opened bei @fbartho. It adds compatibility between TZStackViewAlignment and UIStackViewAlignment so you can easily convert those values in environments where you want to use TZStackView only if UIStackView is not available (< iOS 9.0).

In contrast to #60 this PR fixes the iOS 7 support instead of removing it. Although some features are not available on iOS 7 (like layoutMarginsRelativeArrangement or FirstBaseline alignment) there is no need to completely remove its support.

[toto]

\o/

[fbartho]

I support this PR! -- Have not run it yet.

Minor documentation suggestion: the README.md was originally written differently, I suggest making this change to restore the clarity that this library is explicitly good for iOS 8:

-- ✅ Compatible with **iOS 7.x** or later
+- ✅ Compatible with **iOS 7.x** and **iOS 8.x** or later

Users might not choose a library targeting iOS 7 directly, if they're only trying to support 8 & 9. (It might not be clear if it's relying on deprecated functionality to continue working. -- Because of swift, it isn't!)

[CosynPa]

iOS 7 support is already done by me in PR #59