Bluebird promises support

[iongion]

The then-ability check for promise should be improved to also support Bluebird promises:

        function assertIsAboutPromise(assertion) {
            if (typeof assertion._obj.then !== "function") {
                throw new TypeError(utils.inspect(assertion._obj) + " is not a thenable.");
            }
            if (isLegacyJQueryPromise(assertion._obj)) {
                throw new TypeError("Chai as Promised is incompatible with thenables of jQuery<3.0.0, sorry! Please " +
                                    "upgrade jQuery or use another Promises/A+ compatible library (see " +
                                    "http://promisesaplus.com/).");
            }
        }

...

        function getBasePromise(assertion) {
            return typeof assertion.then === "function" ? assertion : assertion._obj;
        }

The then-ability tests are done in these functions only so a PR might be easy and arrive very soon, what do you think ?

[domenic]

Why do Bluebird promises fail these tests? My understanding is that they have a .then method.

[domenic]

Closing due to lack of response.