Promisify test object creation/destruction

[aanev]

I guess it might be a good idea to promisify object creation/destruction in the tests. For example:

 afterEach(function (done) {
    User.remove().exec(function () {
      Article.remove().exec(done);
    });
  });

can become:

  afterEach(function (done) {
    User.remove().exec()
      .then(Article.remove().exec())
      .then(done());
  });

This can simplify more complex test scenarios where multiple models are used.

[mleanos]

Good idea. A PR would be very welcome for your proposed changes. Thanks you.