Fix for persistent callbacks

[MarshallVielmetti-jhuapl]

My fix for #398

1. Add two private methods to CefQueryCallback_N.java, makePersistent and getIsPersistent, and a private boolean variable persistent_
2. In MessageRouterHandler::OnQuery (message_router_handler.cpp:28), check the value of persistent, if true use JNI_CALL_VOID_METHOD to call makePersistent. This will set a boolean value to true in CefQueryCallback_N
3. In CefQueryCallback_N.cpp::Success, use JNI_CALL_BOOLEAN_METHOD to call getIsPersistent, check the boolean value, and if its true then do not call ClearSelf

I am totally open to fixing anything here, this is my first attempt at open source contribution so please let me know if I'm forgetting something!