Clean up typechecking and related documentation in pymongo.cursor

Currently, fields like skip and limit have typechecking logic in pymongo.cursor. This should be factored out into pymongo.common validation for two reasons: first, it's cleaner. Second, the typechecking in pymongo.cursor is inconsistent (sometimes longs are accepted as well as ints, sometimes only ints are alright) depending on how the field is instantiated.