Add support for constraint identifiers

As discussed in #10.

A few notes:

1. Though `NSLayoutConstraint.identifier` was exposed in iOS 8, it appears to exist as a private method in iOS 7. However, the docs list its availability as added in iOS 7 so its use there should be safe from App Store review.
2. I decided to skip the `#if __IPHONE_OS_VERSION_MAX_ALLOWED >= 80000` dance (which would allow compilation against the iOS 7 SDK). Let me know if you think that’s important.

(Also, credit due to @ericwelander for the original idea many months ago)