Release 0.6.0

• added Type.validate(), which returns validation details
• added support for content: closed
• file-based tests to assert validation details
• API changes:
  • updated Authority.readerFor(): Reader to iteratorFor(): Iterator<IonValue>
  • Schema.getType(name) now returns null when a type isn't found (instead of throwing)
  • removed IonSchemaSystem.getIonSystem()
  • removed IonSchemaSystem.loadSchema(Reader)
  • removed Schema.getType(IonSymbol)