Add methods in RefPointer to allow casting RefPointers of derived classes to RefPoint...

Add methods in RefPointer to allow casting RefPointers of derived classes to RefPointers of their base classes.
This allows for example to implicitly cast a QGst::ElementPtr to a QGst::ObjectPtr without using staticCast()
or dynamicCast(). The use of a static assertion and boost's type traits ensures that you cannot cast
in the opposite direction (i.e. ObjectPtr to ElementPtr).