Added Hbm2Groovy task

For the most part, extends the class structure needed for Hbm2Java.  Used the existing POJO templates with a few modifications (mainly to put the annotations on the fields and remove the getters/setters as Groovy generates them automatically).

Also added a generateConstructors parameter as by default the hbm2groovy task doesn't generate constructors as Groovy creates a named parameter constructor.