XPromise is not A+ Promise compliant

[joelnet]

promises-aplus-tests contains more A+ promise tests which do not pass.

Tests should be converted to use promises-aplus-tests. Code should be refactored so all tests pass.

A pull request with partially completed work is here #1

[joelnet]

This is a known issue. XPromise is not meant to be a production library, but serve as example code for the article https://hackernoon.com/challenge-program-without-variables-javascript-bee89a41455e