Ensure type-level Javadoc in every class, comply with guidelines for
80 char on Javadoc, and minor polish.

- Introcuces XmlEventDecoder which decodes from DataBuffer to
  javax.xml.stream.events.XMLEvent. It uses the Aalto async XML API if
  available, but falls back to a blocking default if not.

- Refacors Jaxb2Decoder to use said XmlEventDecoder, and split the
  stream of events into separate substreams by using the JAXB annotation
  value, one stream for each part of the tree that can be unmarshaled to
  the given type.

- Various improvements in the JAXB code.