Issue 15: Sync issue test case

As discussed in https://github.com/orbitjs/orbit.js/issues/15 the test case for the sync problem. 

Is it enough to directly test the local storage in the returning promise from the rest find call, or is it necessary to wait more? Later, I will also add a test to to make sure that the ``didTransform`` event is fired.

For me, the test case fails at ``equal(localSource.length('planet'), 1, 'local source cache size should == 1');``