Added NestedIDType

This is necessary to support the new type of itemscheme query. Some of the items are NestedNCNameIDType (e.g.: concept and agency), for which the proper type already existed in the schema, but others are NestedIDType (e.g.: category, code, etc.) and this specific type was missing.
Added support for item queries
Added documentation for item queries
Improved documentation of both nested ID types
Added documentation for handling nested id types
Merge pull request #20 from sosna/item-query

Item query - nice work, thumbs up.