Revert Sets().category() from Sets() to Objects()

[nthiery]

In Sage 4.1, the category of a category was changed from Objects() to Sets(). I.e. we used to have:

	sage: Groups().category()
	Category of objects

And now we have:

	sage: Groups().category()
	Category of sets

The attached patch reverts this, in particular because the objects of Sets() are exactly the parents.

[mhansen]

For the record, all tests pass with this applied.

[mhansen]

I think this can go in now and then we'll switch it over to the "Category of Categories" when appropriate.