Changed HubProxy::invoke methods to accept QVariant & QVariantList inste...

...ad of QString & QStringList.  Also added default null value for the HubCallback and a convenience overload of invoke for void(void) calls.

Please consider this slight change.  I think this will allow for richer parameter passing to the hub, without having to stringize everything manually.  For instance, instead of 

    prox->invoke("JoinGroup", "110.110", callback);

you can now do 

    prox->invoke("JoinGroup", 110.110, callback);

ok, maybe that example isn't real impressive :)

Something like this is now possible:

    prox->invoke("JoinGroup", QVariantList() << 110.110 << "foo" << 777, callback);

if you wanted to do such a thing.  You should also be able to pass lists (this is my primary motivation for making the change).  Pretty much anything that QtExtJson can deal with.

Note: My expectation is that this is not a breaking change, and should be fully backwards compatible with existing code.

(Thanks for creating this library! It's been really helpful to me.)