added support for non-XorObject-typed XorReference lists (#2)

- moved Multiplicity and ListItemType properties to XorAttribute base
class (-> XorReferenceAttribute gets new ListItemType property)
- added support for non-XorObject-typed XorReference lists by adding
IEnumerable-cast before setting reference list member
- extended tests to cover new feature
fixed documentation comment after refactoring
new binary release with issue #2 resolved