Added support for reStructured text in admindocs for model docstrings

https://code.djangoproject.com/ticket/5405

Refered to https://github.com/django/django/pull/760 for help
@elvard wrote code to support it, but it needed to be modified, in order to merge
@gkmngrgn added tests that also needed to be modified
Actually, I think it makes more sense to just ignore meta data, as there isn't an obvious usecase for it. It's the same in views - if it's not `Context` or `Template` it's ignored.
I'm afraid it won't look good, if it's just appended to body. Thoughts?