Changed attributes collection to be <string, object> rather than <string, string> to allow arbitrary objects to be attached to a node. See #169.
Removed duplicate ThrowIfReadOnly declaration from AttributeCollection.
Fixed issue with variable not being referenced in AttributeCollection.
Renamed AttributeCollection to AttributeDictionary.
Renamed RouteValueCollection to RouteValueDictionary.
Changed default SiteMapNodeModel display templates to use the Description property as the value of the HTML anchor title attribute for the node link.