Post previews

Hi Jared, 

This PR makes WPs preview functionality work with Timber themes. Will need some refudging when the iterators have been merged, but here it is fully functional with the master branch. Added the test `TimberTest::testGetPostPreview` to show how Timber currently completely ignores previews.

See commit message on d137a69 for details on how it works.

Cheers,
Mike