Add a way to choose between class or package instantiation mode for JaxbContext

[jeremybalan]

I suggest to add a way to choose between class or package instantiation for JaxbContext (class mode is still used by default).

To allow this, the builder of JaxbContextFactory has a new optional method "withJAXBContextInstantiationMode" to provide an enum for the choice between class and package mode. No call to this method = usage of class mode (backward compatible).

Benefits of a package way can be :

• a better usage of the JAXBContext cache : one entry for all class of the package instead of one entry (one JaxbContext) per class
• a usage of the ObjectFactory class with all elements available to be loaded (using class mode needs an analyze to every dependents elements)
• this precedent point can also be a workaround to some problems with class loading mode (for example with Jaxb Moxy implementation, see NullPointerException in AnnotationProcessor.updateGlobalElements() for substituable elements eclipse-ee4j/eclipselink#1856)

[jeremybalan]

Class mode (by default) instantiate a JAXBContext using JAXBContext.newInstance(clazz) as it is actually

And package mode, if chosen, instantiate using JAXBContext.newInstance(clazz.getPackage().getName(), clazz.getClassLoader())