5.2.3-swift3

This is the same release as 5.2.3 below, except it has SWIFT_VERSION=3.0 set in the Xcode project so that Carthage users already using Swift 3 can build it with this configuration.

The sample projects also have been updated to Swift 3 in this branch.

Users of CocoaPods are not impacted by this and can use the main 5.2.3 version — this tag is only useful for Carthage users (until Carthage/#1445 is fixed)