Swift-SDK - 3.1.0

## 3.1.0
July 31, 2019

This release is the GA launch of the Swift SDK. It is a pure Swift implementation which takes complete advantage of language features and performance. It supports Objective-C applications as well.

This modernized version of the SDK is meant to replace the Objective-C SDK.

### New Features
In addition to all features and APIs support of the Objective-C SDK, this new SDK also adds the following new features:

* By default the datafile handler does updates every 10 minutes when the application is in the foreground. To disable this, set the periodicUpdateInterval to zero. If you do allow for polling, the project will update automatically when a new datafile change is received.
* On top of the above functionality, the developer may register for a datafile change notification. This notification will be called anytime a new datafile is downloaded to cache and is used to reinitialize the optimizely client automatically.
* The event handler batches events and will run every minute in the foreground to send events. If there are no events in the queue it will not reschedule.

### Bug Fixes

### Breaking Changes