v3.0.0-beta.3

This release is compatible with Swift 2, Xcode 7 beta 5.

This is a work in progress snapshot of a larger effort to rewrite parts of BrightFutures using Swift 2 features such as error handling and protocol extensions. The tests are still the same and they're all passing. We're keeping an eye on the (syntactical) changes we need to make to the tests and try to keep them reasonable.

• Some free functions have moved to protocol extensions (e.g. flatten(f) now is f.flatten())
• success, failure and complete now throw an error if the Future was already completed
• Static methods used to create Futures (e.g. Future.failed) have been replaced with initializers