Configurable xjc main class

added new setting xjcMainClass that allows configuration of the class to run for the xjc task.  Doing this because older versions of xjc have a different class they run off of.  2.0.x use com.sun.tools.xjc.XJCFacade, while 1.x use com.sun.tools.xjc.Driver.

Setting defaults to previously hard-coded value: "com.sun.tools.xjc.XJCFacade" so no behavior change unless explicitly configured.