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 don't think we need to preserve the `_class` names. The public API is the two configurable traitlets that you're working with here.