Fixup TZStackViewAlignment

[fbartho]

This PR updates TZStackViewAlignment to match UIStackViewAlignment and has some minor updates for iOS deprecations / consistency in documentation & test project.

Background

We noticed some discrepancies between the functionality of iOS 9.x UIStackView and iOS 8.x TZStackView. Specifically, we noticed that the following did not work as expected:

stack.alignment = UIStackViewAlignmentCenter;

Things discovered:

1. TZStackViewAlignment had a different order of enum values than UIStackViewAlignment. This mismatch meant that the above assignment didn't go as planned.
2. The code was using deprecated methods addConstraint()/addConstraints()
3. The code was using other methods that required 8.x support.
4. The xcodeproj was set to deployment target of 8.0, but the README.md claimed 7.x support.

These were all brought to consistency in this PR, along with using NSLayoutConstraint.activateConstraints()

[fbartho]

Please see #63 instead of this PR if iOS 7 support is important.