ABDERA-352 @Extension annotated List<> 

Used the same system in BaseSerializer.writeExtensions() as in BaseSerializer.writeElements() with calling BaseSerializer.toArray() and looping through the values.

@Extension annotated List<> of object now get serialized into multiple elements.
See extensions/serializer/src/test/java/org/apache/abdera/test/ext/serializer/ExtensionAnnotationTest.java for details.