Pre-release 4, improvements for custom test action composition

The API has been refactored to take full advantage of the sequential nature of KIF-next to compose test steps as chains of simple commands which can be more easily swapped out.

Pre-release 3, improved API for extensions

This release enhances support for the latest iOS build (makes the tests pass, no major changes) and improves the API for extensions, making it easier to raise errors, validate test steps, etc.