2.0.0

New major release. The goal was to streamline the API and to make it more compliant with other promise libraries and especially with the new upcoming ES6 promises specification.

• Add standalone Promise class.
• Add new React\Promise\race() function.
• BC break: Bump minimum PHP version to PHP 5.4.
• BC break: Remove ResolverInterface and PromiseInterface from Deferred.
• BC break: Change signature of PromiseInterface.
• BC break: Remove When and Util classes and move static methods to functions.
• BC break: FulfilledPromise and RejectedPromise now throw an exception when initialized with a promise instead of a value/reason.
• BC break: React\Promise\Deferred::resolve() and React\Promise\Deferred::reject() no longer return a promise.