Add new helper visitAndRespond to fix issues with Ember >= 1.4.0.

Details from #10:

1. The `wait` helper in `ember-testing` is intended to wait for the
routing transition to be completed (See
[here](https://github.com/emberjs/ember.js/blob/master/packages_es6/ember-testing/lib/helpers.js#L146-148)).
2. `httpRespond` is setup via `registerAsyncHelper`.
3. An async helper from the perspective of the `ember-testing` package
chains off of the previous promise.
4. Because of the above, `httpRespond` never gets a chance to respond,
and causes the test to hang (waiting on the router to resolve its
promise which will never happen because httpRespond was responsible for
resolving it).

The reason this worked up until
https://github.com/emberjs/ember.js/pull/4347, was that the router was
completely refactored (prior to 1.0.0-rc.6) and the flag that
`ember-testing` was previously checking `router.isLoading` was NEVER set
so it was always false (and therefore would never wait for the routers
transition to resolve).  The change made in that PR was a good change,
and brought the `wait` helper back inline with what I assume was its
intent all along.

---------

OK, all that said, I have a solution, but it isn't API compatible with
the prior versions.  Basically, we need to have the `visit` AND the
`httpRespond` bundled together in a single async helper.  This allows us
to kick off the router transition, and then handle the canned response
before calling `wait` (and only `wait`'ing at the end of both).

This may or may not be an acceptable solution to @trek, but it should
work and resolve the issue reported here.
Merge pull request #13 from rjackson/fixish-1-4+

Add new helper visitAndRespond to fix issues with Ember >= 1.4.0.