Clarify mocking of window.Promise

[cburgmer]

I'm using es6-promise.auto.js to provide a shim when testing with PhantomJS. It wasn't obvious which method of mocking applied to me, and as for the built-in Promise, it was unclear that it's up to me to "hot swap" the constructor.

Right now I'm doing

    beforeEach(function () {
        window.Promise = mockPromises.getMockPromise(Promise);
    });

    afterEach(function () {
        window.Promise = mockPromises.getOriginalPromise();
    });

Calling this example out would help a newbie like me :)

[charleshansen]

Sure, I'll add something. Glad it worked for you.