finally(callback) {
    let promise = this;
    let constructor = promise.constructor;

    if ( isFunction(callback) ) {
      return promise.then(value => constructor.resolve(callback()).then(() => value),
                         reason => constructor.resolve(callback()).then(() => { throw reason; }));
    }

    return promise.then(callback, callback);
  }
`finally` will be invoked regardless of the promise's fate just as native
  try/catch/finally behaves

  Synchronous example:

  ```js
  findAuthor() {
    if (Math.random() > 0.5) {
      throw new Error();
    }
    return new Author();
  }

  try {
    return findAuthor(); // succeed or fail
  } catch(error) {
    return findOtherAuther();
  } finally {
    // always runs
    // doesn't affect the return value
  }
  ```

  Asynchronous example:

  ```js
  findAuthor().catch(function(reason){
    return findOtherAuther();
  }).finally(function(){
    // author was either found, or not
  });
  ```

  @method finally
  @param {Function} callback
  @return {Promise}