Refactor GuiceObjectFactory and avoid creating unnecessary injectors

I did some refactoring to make it more simple and and easier to understand. I also removed the unnecessary Injector that was created if no Injector was found in the servlet context.

Other than that it looks fine to me!
I don't know if there's any chance the AtmosphereConfig parameter could be null, but it's better to fail immediately than to fail in AtmosphereModule when injecting something.