Add support for ordering with `OrderBy` or `F`

[AndreyMZ]

There are many way in Django to specify an ordering by column:

	ASC	DESC
str	'my_order'	'-my_order'
F	F('my_order')
OrderBy	F('my_order').asc()	F('my_order').desc()
OrderBy	OrderBy(F('my_order'))	OrderBy(F('my_order'), descending=True)

This PR adds support for all this ways. Now one can write:

class UpOrderedSortableBookAdmin(SortableBookAdmin):
    ordering = [F('my_order').asc()]

class DownOrderedSortableBookAdmin(SortableBookAdmin):
    ordering = [F('my_order').desc()]

Also custom ChangeList.get_ordering may return instances of OrderBy or F, not only str. Previously this resulted in e.g.:

AttributeError: 'OrderBy' object has no attribute 'rpartition'

---

For the rest minor changes see the commit messages.

[jrief]

Shall I release it as 2.2?

Thanks for this PR. Actually, I prefer smaller PRs which do not mix up fixes and features and version bumps, but I'm going to merge this anyway.

[AndreyMZ]

Shall I release it as 2.2?

I would appreciate it. Thank you!