Add an option to run all tests with the 'unicode_literals' option

@taldcroft - this is for you :)

```
python setup.py test --unicode-literals
```

will now make sure that all tests are also run with the ``unicode_literals`` import from ``__future__``

This is a very primitive solution, but it seems to work (though of course, some tests are failing, and on my machine one of the FITS tests segfaults).

cc @mdboom @embray

* [ ] make option available for ``test()`` function too
* [ ] remove ``unicode_literals`` import from current tests to ensure that we always test both cases?
* [ ] should this option be enabled by default in Python 2?