Fixes Swift tests to compile under Swift 3 to make Carthage work again.

Implements default signal types as concrete classes to enhance Swift compatibility. Adds nullability identifiers throughout.

Adds alternate method to create concrete subclasses of signals in order to be able to bridge Signals to Swift.

Adds support for tvOS and watchOS.

Bumped back podspec to support iOS 7.0

Augments the fire-signature of an empty signal from (void (^)())fire; to (void (^)(void))fire; to give a compiler error when users accidentally add data to an empty fire.

Deprecating init in favor of always calling initWithProtocol. Bumped major version due to breaking nature of this change.