-cherry picking from master to get jaxbtask working and compiling on branch

Conflicts:
jbpm-human-task/jbpm-human-task-core/src/main/java/org/jbpm/services/tas
k/impl/model/xml/JaxbTask.java
jbpm-human-task/jbpm-human-task-core/src/test/java/org/jbpm/services/tas
k/jaxb/AbstractTaskSerializationTest.java