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
I don't think `Context` or `Template` (as for views) are good examples for meta data on a model. And I can't think of a use-case right now. Is it possible to render everything after the first "line" (summary) as body? That would probably make more sense.