Added custom names to identify MBassador dispatchers.

Added custom names to identify MBassador dispatchers.
I also added a null check on ExecutorService.

I didn't want to touch any of the code that you mentioned in #45 you were going to investigate, but I wanted to request that you give custom names to threads (or, ideally, provide functionality for the user to name them via ThreadFactory). This can be very helpful with debugging a multithreaded application.