Allow generated SVUID field to be marked synthetic

Apache Aries proxies need to be able to mark generated svuid fields as
synthetic.  I think this might generally be a useful feature and is easier to
add to the SerialVersionUIDAdder than to add on with delegation/subclassing. 
If you don't like this idea can you at least add a getter for the hasSVUID
field (it was protected in asm 3, thus accessible to subclasses).

The attached patch adds the optional capability to mark added SerialVersionUID
fields synthetic and enhances the test to check whether the SerialVersionUID
field is actually added when expected and if the added field is synthetic as
configured.