clean up ChangeRecord API

[jmesserly]

moved from https://code.google.com/p/dart/issues/detail?id=14157

• Observable.changes should only be property changes.
• ObservableList will have "listChanges" property which includes the ListChangeRecords.
  Splitting this allows the consumer to know what kind of records it is getting, and makes the API constraints more clear.
• remove ChangeRecord base class
• add "target" to List/Map change record
• add old/new values to ListChangeRecord? See http://wiki.ecmascript.org/doku.php?id=harmony:observe#array.observe