SimplePromise: Simplified version of Javascript's Promise class

this class implements the following methods:

• new Promise(executor)
• Promise.all(iterable)
• Promise.resolve(reason)
• Promise.reject(value)
• Promise.prototype.then(func)
• Promise.prototype.catch(func)
• Promise.prototype.finally(func)

all the specification of Promise can be found here.

Note: The code is not transpiled into ES5 so it may not run on outdated browsers.