v4.0.0: Multi-user support

Version 4.0.0 adds the ability to pass customer authentication to Apptentive to protect and separate each customer's personal information from one another. It uses a new method for initializing the SDK. There is also a new method (required for using push) to forward local notifications to the SDK. Static library support has been dropped in favor of a dynamic framework. There is a new runtime setting for log level, which defaults to INFO in all build configurations.