Adds is_xblock_visible_to_students

Currently only checks release dates because staff lock doesn't exist yet.

@cahrens @andy-armstrong @nasthagiri 
I would suggest a helper method to simplify these test points. Something like

```
def _create_xblock_with_start_date(name, start_date):
    location = Location('edX', 'visibility', '2012_Fall', 'vertical', name)
    vertical = modulestore().create_xmodule(location)
    vertical.start = start_date
    modulestore().update_item(vertical, self.dummy_user, allow_not_found=True)
    return vertical
```

Note that the name really doesn't matter-- you could use the same name for all and not pass it in.