Change ``_get_rendered`` behavior for automatic generation of rendered

Duaring loaddata, ``pre_save`` method is not called that's why if you
want to create DEMO value or whatever with Fixture, empty field is
created.

To slove this issue, I change ``_get_rendered`` behavior to generate
rendered text automatically when stored rendered text is empty.

With this change, you can use fixture to create (but not update) field.
Converting self.rendered to unicode, so that __unicode__ returns unicode
Merge https://github.com/grahamking/django-markupfield
Pull in lambdalisue's pull request #8