MAINT: Use `Type` instead of `DottedObjectName` for managers.

`Type` is strictly more powerful than `DottedObjectName` and is easier for users to override.

This specifically came up in the context of wanting to pass a class object for `contents_manager_class`, but it seemed reasonable to change the rest of the managers for consistency.

Ported from https://github.com/minrk/ipython/pull/21.
ping @minrk 
I would rename the configurables to `_class` and remove this assignment. These being configurable is new in 3.0, so I'd rather it be consistent with the rest of the configurable classes.